I thought steam cracked down on this kind of betting in the last year and banned a bunch of people who were involved? It seems they don't like these gambling sites, they can be easily rigged against you and even if you did win - you might get a steam ban.
Well steam came out and shut down a few of the bigger gambling sites a few months ago. Since then, most sites have adopted a coin/point system. When your skins are deposited, they are converted to an on site currency, which Is perfectly fine. I think steam more so just wanted to detach themselves from the gambling scene as much as they could.