Alice,
I think there may be a couple of factors at work here. A lot of companies are prohibiting affiliates from using their name or any trademarked terms in PPC advertising. It seems pretty dumb to me, but no doubt their lawyers have come up with a raft of reasons why it may be a good idea to control the way these terms are used.
Secondly - and more worryingly in my opinion - Overture has been lobbied by (I believe) drugstore.com to ban all advertising of prescription drugs except by "licensed pharmacies", even though the selling of those drugs online is perfectly legal. Strangely, it seems that drugstore.com (f that's who it is) is one of the very few "licensed pharmacies" around!
Overture agreed to ban all drug terms last month, and I believe that Google said they would follow suit shortly. As a Brit, I probably have no right to vent about the state of the US health system and the deliberately inflated price of drugs, so I won't - but I do strongly object to the level of censorship that corporations are allowed to impose in order to protect their profits!
I will now retire to my dragon's lair, breathing fire
Isobel