Apparently a lot of pharmaceutical research goes into making controlled substances in pill form difficult to grind up and snort for a "high", why not just cut the filler in the pill with black pepper?

Allergy concerns. Possible reactions with whatever is in the pill that's supposed to work - you can't just mix it with anything, there's a reason sometimes one brand of medicine works better for a person than others even if the active ingredient and dosage are the same. Probably a lot more problems someone with actual expertise in this stuff can point out.

People will successfully snort black pepper, especially when drugs are involved

I figure there's gotta be a good reason or they would have done it already, just seems like such an obvious solution