I do not know this for sure, but lets think about it from the Sites POV.
They are, for the most part, trying to promotes an honest image. So they become the de fact cop involved in scrutinizing all withdrawals.
They will want to guard against money laundering, obviously, or becoming an off the wall (and off the grid) money transferring service, partly because it would be very possible to get money into the hands of bad folks, like terrorists for example. Not to mention a gazillion tax evasion scams that might be possible. The best argument for the UIGEA is that it was too easy to move money to the bad guys. No site will want to be causing headaches for itself.
Those, along with lots of other less than honest endeavors will trigger alarms that will cause you problems trying to use the sites to transfer money only to withdraw it immediately.
Again, I can't be absolutely sure, but don't count on it working that easy.
All that said, if you are an established long time player at a particular site, I would guess it becomes a lot easier.