I love that they technically let people do this, even though it seems like a "loophole" ripe for abuse. It's a nice attitude.
(For those who don't know – Heroku gives you 750 free dyno-hours per app per month, forever. This is in contrast to AWS, which gives you 750 free instance-hours per month per account and only for the first year, AFAIK.)
It's worth noting that Heroku silently spins down free-level apps that aren't in use (restarting them on a request), so they aren't necessarily consuming all 750hrs of machine time on every free app.
(For those who don't know – Heroku gives you 750 free dyno-hours per app per month, forever. This is in contrast to AWS, which gives you 750 free instance-hours per month per account and only for the first year, AFAIK.)