I believe it was written into the concession with Stagecoach that SYPTE have to treat trams equally with buses.
That would make sense given that there are now no bus services at all on certain parts of the tram route (notably Park Grange Road).
It's also important that passes are accepted on Supertram as the Supertramlink ("the bus that thinks it's a tram") goes into Derbyshire council area and it's susposed to work as part of the network.
Blackpool used to offer it as well but recently withdrew in.