I do not believe the US Government is a giant conspiracy to evade the constitution for the purpose of apprehending drug users. I believe that when the government seeks a warrant to surveil foreign targets for counterterrorism, their interests are actually in disrupting terrorism.
That doesn't mean I think we have effective counterterrorism (for instance, there's widespread evidence that CIA uses, or at least for a long time used, torture to attempt to obtain information in counterterrorism cases; torture is morally repellent and, equally importantly, demonstrably counterproductive), or that I think terrorism is legitimately the key federal goal that DHS and CIA claim that it is.
But I don't subscribe to the slippery slope argument that suggests that the government will inevitably use every power we give it for any purpose to, I don't know, enforce the Comics Code Authority.
Aside from what tptacek has to say, there's generally a strong distinction between intelligence and law enforcement that's more or less enforced by the exclusionary rule. Ironically, there might actually be benefits to outright warrantless wiretapping as opposed to use of the FISA court to obtain warrants, namely that any such wiretaps could only be used for intelligence purposes, never for law enforcement.
