Its not high risk, but they can seize money associated with the advertising.
"The Sporting News (TSN) took its time in stopping the ads (six months), which pissed off the government, which three years later fined them $7.2 million, in January 2006. The settlement represents TSN's profits on the ads for the three years it accepted them (2000-2003), and was paid with $4.2 million in cash, and $3 million in public service ads in its own publications explaining how online gambling is illegal. (more from the Corporate Crime Reporter, the Olympian, and Media Buyer Planner)
In December 2007 the three big Internet giants paid a fine for accepting ads for online gaming. It's not even clear that that was illegal, but the companies evidently felt it was simpler just to settle. Google's fine was only about a third of a single day's profit. (Point-Spreads.com) The three companies had actually ceased taking online gambling ads about four years before the settlement, when they got the warning, but the feds fined them anyway -- years later."
http://vegasclick.com/online/legal.html