We are actually considering right now whether to allow "negative" promotions in our affiliate campaigns. All the above are valid points, meaning that the user is going to be searching for it anyways, so might as well have a "friendly" site show up for the rankings.

What we are actually thinking of doing (though this may get too complicated for some)
is allow "negative" review/domains, however no negative promotion via PPC.

Meaning if a user actually searches for "Product Name Scam"
then they can find the natural search results.. however what I DONT want to see is the user searching for
"Product Name"

and seeing a blaring PPC ad that says "Product Name Scam"
These sort of things leave impressions you know..

Do you think that will too complicated for affiliates to comprehend and adhere to?

ie.. no Negative promotion in PPC?