You do not require 56% hash to double spend, you need to use certain programs that allows you to send your money to two different places at the same time which until confirmation doesn't mean that you lost your money just yet, you need to lose your money only if there is a confirmation, if you use zero satoshi fee or anything similar like that usually you end up with gambling way before you get any confirmation, sure there are risks involved but you are trying to cheat a casino so you do take the risks involved with that as well.
Doesn't mean it will work all the time but at least it is definitely higher chance than just going to a casino and trying to win. Double spenders and people with multiple accounts could try to say anything they want to look like they didn't do anything wrong however the facts are facts and rules are rules so there is no going back on these things when you are caught doing something against the rules just once.
It's 51%, but actually, doublespending attacks on casinos are far easier to do.
You're right, but to explain a bit better... Basically, it only affects casinos that accept deposits on 0 confirmations, ie. as soon as the transaction is broadcast and detected by the casino's wallet.
1. Guy deposits Bitcoin with very low fee. Meanwhile he makes a second tx with the same bitcoin to himself, but with a higher fee.
2. Casino detects first tx and credits guy's balance.
3. Guy bets it all. If he wins. He withdraws. If he loses, he has his double spend to confirm with a higher fee.