Venues that host tribute bands still need to PRS, however some tribute bands or acts have been forced to stop due to estates. They either want a cut or the lawyers get involved.
I knew a Blues Brothers pub tribute act. They were very good but had to stop because the John Belushi estate wanted a cut or the lawyers would have gotten involved. Even though the venue paid PRS. Some suits from PRS do scout venues to make sure that venue owners are sticking to the laws.
Some venues either want 10% of each item sold at concessions or take out the agreed percentage at the end of the night.
www.prsformusic.com/royalties/live-performance-royalties