They've been around for a long time.
They were featured on Something Awful's Weekend Web back in 2009
http://www.somethingawful.com/weekend-web/beastforum/1/
and Breitbart did a piece on them in 2015
http://www.breitbart.com/london/2015/01/29/inside-the-sick-secret-world-of-online-bestiality-forums/
They don't just talk about it, though. They upload images/videos of it (to the extent that if you GIS "beastforum" the images returned will be bestiality porn) and they use the forum to arrange real life meetups for animal sex (yes, Virginia, pimping out your husky for sex with humans is a thing).
That they don't get prosecuted isn't what's especially strange, but you'd think they'd have a wee bit of trouble maintaining hosting given that they're a high profile site and it doesn't take much for people to complain to hosts.