Web Recommendations Systems

So, I blogged previously about how I love Netflix's recommendations system, and it's a lot like Amazon's as well.

I wonder why these sites (along with others that do recommendations) only mark things as recommended — why don't they tell me if I probably won't like something I'm looking at?

I understand it in the case of Amazon — it's in their interest to sell me everything. On the other hand, it's in Netflix's interest to have me not check out a movie.