I have one question for you:
Did you bet more than 6mbtc per spin while you had an active bonus?
If so then your shit out of luck buddy. The term is clearly laid out on their website in the Bonus terms and conditions. Whether you think its ridiculous or not if you broke the term while using the bonus to obtain your 20BTC balance then you clearly have no case. It's up to you the player to carefully read any terms and conditions when you accept a bonus at any online casino. Most have a term in place that limit the amount you can bet while using an active bonus. And at most fiat online casinos this amount is usually much less and is also very common practice.
Next time you take a bonus at any online casino read the terms so you don't break the rules again. This was a very costly mistake you made yourself.
First of all, illegal casinos can not have valid terms and conditions and therefore he could not breach any.
Secondly, if he bet more than the bonus rule states, why do they ask for his documents? Why not simply say that he bet more than allowed and that's it?
Why always this KYC nonsense if someone won a larger amount?
If the identity of the player is so important (for a bitcoin casino), why do they not check it prior to accepting any deposits or bonus?
If he has lost, his identity of course does not play any role!
Just to clarify, if minors are not allowed to enter a disco, they logically check it at the entrance and not at the exit!