This is allowed.
I am not sure why would they block your 20$ withdrawal, it doesn't make sense, unless their system has found something and you are not being honest.

One can hardly make any conclusion from this topic, but it would be nice to hear other side too.
My main complain is (if it wasn't well described in the post above): why was I even able to deposit and to use the account normally, and just when trying to withdraw it was shown the message?
This question has been asked million times for different casinos, withdrawal will "trigger alarm" - deposit won't, that is the way how 99% gambling sites are working and no matter what you do you can't change this fact.