I wouldn't doubt that Coinbase just put that rule in place so they could lock people's accounts to gain more Bitcoin. It probably has more to do with making money than it has to do with actually gambling with Bitcoins.
I don't think they are seizing funds, just refusing to do business with the person, and if I'm not mistaken asking them to withdraw their bitcoin.
IMO not a money-grab by Coinbase, simply their directors following the advice of their lawyers.