Obviously they can rescind the pre-offer if things turn up in your past from PRIA or background checks.

Everything is conditional until your drug test is done.

I start later this month and while my PRIA and background checks are complete, it's still conditional until my drug test is done.

Personally I don't think it's a bad thing that they're turning people away that have skeletons in the closet that aren't reasonably explained.

I may not be technically correct as to what is conditional upon what, but it gives you a fair idea of the process I just went through.