Most girls can't do it, but many can with some practice.
It's not piss, it's a super wet pussy from proper g-pot stimulation. Mixed that with a girls orgasm that is so much longer and intense, the horny juice builds and they are able to push the liquid out with force and/or the body starts to spasm and the pressure bursts the liquid out.
I have no idea how big the market for it is. My guess is, most guys can't figure out how to get a girl to do it, so it's just piss to them.
