In their terms and conditions they pretty much say they can (but are not obliged to) do whatever they want to when they receive a submitted review or when they receive a complaint about a posted review. The results of their vetting may therefore be a little uneven, even though by now they probably have pretty detailed guidelines for the overseers to follow.
Sometimes it's absurd. They caution you not to use crude language but then they mess up and asterisk out stuff like the Latin word for "with" in some phrase, because out of context it is a crude English word. Also, it can take quite awhile for them to remove an offensive comment even if it's pretty blatant.
I wrote a couple times to complain about one review, two months apart. I think the second time I was exasperated and added something like "If Steve Jobs saw this post, he'd have your &ss." Finally they took it down. As they tell you up front, they're not obliged to do so. Still, if some review is way out of line and you write in to complain, they will take it down.