The only reasons I can think of why a pub owner would object to a tip jar would be:

1) He/she might feel the tips would cut into the waitstaff's tips

2) He/she might feel that patrons would feel obligated to pay for entertainment when the performer is already being paid a fee - so the performer would be "double dipping" and some patrons might resent the fact that the entertainment was not free.

However as long as the tip jar is just out there and the performer is not begging for money I don't see why it should be a problem. I think making humorous references to the tip jar is okay (just so people will know it's there) as long as it isn't overdone.