It sounds very odd - how can they insist that you "attend" without paying you?
There are three options as always: accept it, negotiate, or vote with your feet. Personally I would go for the second and break down the us/them barrier that seems like it may be present - such a barrier doesn't help any of the stakeholders and belongs to the sixties.
Make sure you maintain a positive attitude (avoid wingeing); some of the things you may want to propose are a standby rota system or attendance pay.
HFD