I've never seen Stake or Primedice require documents for large withdraws/wins, have I missed something? I've never played on either site and I'm not familiar with that but this isn't something I would expect them to do.
I guess will never know since it is stated that they only asked if you win already a large amount (which not yet confirmed if they really implement KYC ) but in ICO it seems mandatory now since many cheaters and hackers were already around. The only downside if ever in a casino is that they should inform their players in advance or make an announcement of it before their players decide to deposit some amounts on them that there are some regulations like that since there are people who want to keep themselves anonymous.