Its KYC and a lot of sites have it, and honestly, I don't understand it either as these gambling sites employ bitcoin for gambling, and I am almost certain they themselves don't pay their taxes to the correct amount themselves.
it is not about taxes or security or whatever reason they are claiming it is
the reasons behind the checks are simple:money or another pretext to not pay you your winning
once again,they never protest or ask you to submit ID verification when you DEPOSIT
this should have been done this way: you register,then you ask the site to check if everything is alright with you and your account before you deposit money
but this is only a dream,in reality they would freeze your balance under any pretext and tell you tales of security checks and that your coins are tainted,or they detected irregularities in your bet pattern or the shoe size
any frigging reason to not pay they will use,some people give up and do not send their ID so its just an easy way to save money for them
and nowadays bitcoin aka anonymous gambling is almost nothing different from the regular casinos