By the way this question was just recently posted on the San Diego board and there was some good responses. I would post the link to that discussion but I'm too lazy right now

however it is still really near the top of the page if you want to check it out.
Anyway I think the two main reasons are:
1) the provider places the ad, and the ad mall automatically links their reviews. I believe some ad malls do this and the provider is not even aware that her bad reviews are going to be linked to her ad.
2) some providers are counting on the hobbyist being lazy and not checking. If they see "check out my reviews," for some (foolish) guys that is enough. They figure she had reviews, she must be ok. They don't bother to check that the reviews are bad.