A lot of people don't, it would appear from reading this post on Joan Eisenstodt's blog. She outlines just a few of the ethically challenged scenarios planners and hospitality partners face all the time, and notes that many don't seem to understand why that particular carrot (say, you get an iPod for booking business with us) is a little on the slimy side.
The biggest issue, as I noted in a comment there, is that, ethical or not, these come-ons work, and not all just because some people don't understand why certain practices are wrong. I think a lot of people know it's wrong and do it anyway--take that fam to Hawaii even if you'll never plan a meeting there, or book that hotel over another for personal points--maybe as a reward for other parts of the job that seem unfair, kind of as a way to balance it out. But that doesn't make it right.
So the problem is two-fold: Educating those who really don't get why it's wrong to profit personally from a business decision, and trying to get those who get the idea but somehow can justify it to themselves to cut it out. The former will be infinitely easier to do than the latter, I'm afraid.