Each site differs when it comes to requirements on withdrawing bonus money. These usually involve things like minimum points/hands earned, then having to go through security/identity checks to get it out.
I suppose if somebody did want to go through all that trouble to possibly double their money then i guess it can be done.
What usually happens though is the player loses most or all of their initial deposit and continues playing on the site with the bonus money just to get their initial deposit back. Some get it back and make a happy withdrawal, but most just go broke, so the sites have no problem recouping the bonus money.