It's very common in Vegas. At some hotels, an employee who accepts a bribe for an upgrade is probably in violation of the rules given by hotel management. At many hotels, it's an accepted practice allowed by management.
I don't know what the actual policies are at the Westin in Las Vegas.
This is the Las Vegas forum's main thread on the topic:
http://www.flyertalk.com/forum/las-v...very-well.html