That's an absurd restriction that would waste a vast amount of public money only to defeat the entire point of legalisation.
Most people still won't do drugs even if they were legal, and of the ones who do, most will obey any restriction you put on them. So you'll spend billions in public money testing everyone who needs any kind of social assistance, only to give about 95% of them their money anyway.
Of course, some will fail. That doesn't necessarily mean they're an addict, any more than a glass of champagne at New Years makes you an alcoholic. Many will be occasional users who will see no benefit whatsoever from enforced rehab. That's taking valuable rehab spaces away from those who genuinely need them. Oh, and when you're forcing treatment on the poorest members of society, they probably can't pay for it themselves. That's more money from the public purse.
Then you've got new applicants who've just lost their job. Are they sent to enforced rehab, or just denied assistance until they can pass a drug test?
Some will be addicts. Unless they actually want to quit, rehab won't fix that. And the ones who want drugs are going to get drugs anyway. You're not going to stop that just by removing their money. What you will do is reinstate the very black market you just got rid of through legalisation.
Which brings us to the deeper problem with your plan. If you remove a persons ability to pay for food and shelter, what do you believe that person will do? They're certainly not just going to crawl into the woods and die quietly - they'll do what they need to do to survive.
Social assistance doesn't just help the person receiving it - it helps society as a whole by removing the need to turn to crime just so you can eat.
This is the problem with the drug testing argument, the slippery slope leads into caste oriented nanny state territory. Such an argument could conceivably go even further.
EG: Ban any "unhealthy" food from being purchased using public assistance. Require anyone who is on public assistance provide proof of doing 30 minutes of daily exercise. Tie public assistance aid to your credit score. Place monitors in the house so if one is engaging in too much time-wasting activity (video game binges! hours of social media!) your funding is cut.
This could get pretty nightmarish frankly in an Orwellian way, especially if we combine this with the "world is so automated a basic income is absolutely necessary" idea that's floated around here a lot.
Generally, in the states that drug test for "welfare", the results seem to mostly point to a waste of money (typical articles on the phenomenon are here: http://www.forbes.com/sites/judystone/2015/02/17/the-sham-of...). As my above point on reagents, the standard "pee test" involves similar principals; the only way to truly confirm a positive pee test is to send it to a lab for GCMS analysis. This is not cheap.
I feel like it's actually pretty easy to avoid any slippery slope. If you are accepting money from the government, there should be nothing you are spending money on that isn't explicitly sanctioned by the government. I as a taxpayer am not paying for your food so that you can spend the paltry sum you earn on cigarettes.
Also, I don't really the see problem with a "nanny state" in the context of people that are accepting money from the government. If you need the government to provide for you, you should be totally happy with whatever stipulations that help includes.
Let's say the government proposed a rule tying Medicare type payments to your BMI (gradated until >30 BMI = no more Medicare pending graduation from a "weight loss bootcamp"). I can pretty much guarantee a huge amount of people would be totally not happy.
I could use similar harsh, if negatively over-generic, logic to justify the rule ("I'm not paying for the health costs of some fat ass who ate so many donuts in their life they're now having a heart attack on my dime!"). But such an approach will probably not solve very much anyways except make a whole lot of people angry. People would probably hold donut-eating parties in protest. :)
One other point. At present, the current state legislative trend on drug-testing "welfare" recipients is mostly limited to TANF, with occasional forays into Medicaid and SNAP. I have seen no proposals targeting SSDI / Social Security or Medicare. There's a reason I said "caste" -- right now, the context is not people in general that are accepting money from the government, but only the "wrong" people that are accepting money from the government.
How do you define "accepting money from the government"? I assume you're mostly referring to SNAP, TANF, and Section 8 but where do you draw your ideological line? Anyone accessing any Federal Assistance? Does SSI count? Tax credits? Pell Grants? Should drug users be denied access to tax funded infrastructure and emergency services?
If feasibly testable, then yes. If you need the government to help you pay your food/medical/housing/whatever bills, you should not be buying alcohol/cigarettes. Caffeine is debatable, because for many people it's not about recreation, it's more about giving you that extra kick so that you're more productive.
I feel like the best thing to do in regards to drug legalization is to try to empirically link each class/type of drug to various societal outputs, like productivity, price of medical care, etc, and then tax the drugs accordingly.
Meth decrease general productivity by 100%? Tax it at a higher rate. Cocaine/MDMA/LSD have benefits and fewer side affects? Tax them at lower rates.
If you are on food stamps, there is absolutely no reason you should be spending money on any type of recreational drug.
So mandatory drug tests before any social spending.
We'd need to get the cost down and efficacy up for drug testing, though.
If you are destitute and they find drugs in your system, you are sent to rehab.