Well, I'll say a couple of things about them, and leave my personal feelings out of it
The legality of these sites is such that in 26 (was 24 until recently) states there is no direct law prohibiting it. Most of the laws on the different states books which do pertain to it are worded in ways that reference unnatural acts, which is a very broad brush and can include much more than these specific acts.
And no, Customs won't be prosecuting anyone for it under the current system, and it would take laws being made on the federal level in order for them to get involved. That's straight from the top so to speak.
Now before everyone runs off to try and start a site let me bring up one point --
Porn prosecutions aren't made on what positions are legal or not, or whether or not its ok to have sex with someone you aren't married to.
Prosecution comes in the guise of obscenity most of the time, and I don't know how many locations you will find in the US where a dog getting a blowjob from some chick isn't going to be found obscene.
If you are a paysite owner (or a freesite owner as well) I would suggest taking your cue from the large program tours as to what you can most likely get away with and not. These are the guys who've got the money and spend it to hire attorneys to advise them on where to draw the line with tours and types of sites to push. That or hire a lawyer of your own to advise you in what is definitely a grey area