Ok I'm going to justify my answer with a statement full of speculation... Here goes!

Mentats function by speeding up the mental processes and boosting memory related functions (http://fallout.wikia.com/wiki/Mentats). Now while this could be considered cheating by some, all they really do is make the brain more efficient. The pills can't make the user pull answers out of thin air, so the user would need to already have absorbed the needed information at some point (you'd be surprised at how much information your brain can take in without you knowing). Functioning solely as a mental stimulant, like caffeine, it seems logical to assume that Mentats were accepted.

Steroids force the body to do something whereas Mentats would simply aid in the access of latent information. That's why I think they weren't banned from colleges. Besides, Med-Tek (the maker of the drug) was a large company that would've been more than capable of lobbying their Congress (who probably relied heavily on the drug) to support them.

I think that makes sense. I just woke up and I haven't finished my first cup of coffee yet.