Re taking liberties....a good cover or tribute band will put in ad libs and to a certain extent put their own stamp on the performance...however in this BUSINESS the fickle public have to be entertained and pleased so they have to be provided with what is expected. It is all about striking a balance and fulfilling obligations.
Sadly all sorts of restrictions apply to what a pro band can get away with. At the end of the day it is A BUSINESS and only a few can get away with breaking the rules or solely playing what THEY want to play all the time.