I think in the past they are allowing, but now that they see the risk, they don't allow it anymore. The risk here is that, once they make a deposit, they can put a little fee so it will not confirm right away, gamble the credits and if that bet will lose, they will just double spend it, that's so clever from the gambler's side so gambling sites have to also adjust.
At this stage that blockchain is congested, 1 confirmation sometimes takes too long if you don't put enough fee, so it would still not help you if you are in a rush of gambling, better use altcoins instead.
Most gambling website may not have that same feature that credits your deposits right away even without 1 confirmation.
It could be abused.
He can use Bitcoin to gamble even If he's rushing anyway, just check the mempool traffic and fees. If tx fees are high then that will be the time to use either of the altcoins.
Last week was a hell for all Bitcoin owners because the fees are extremely high caused by the black out in China, but right now medium fee ranges only at $1 ~ $3. So, that means you could transfer Bitcoin to gambling website or vice versa at lower priority and gets confirmed after a few minutes to one hour.