Well, it's not an unusual arrangement. Most hospitals I know of ask their nurses to participate in a committee or some other group that works to "better" the general work environment or a unit's specific work environment. It's not strange.
I understand you like doing your thing on the weekend, then leaving the hospital behind to have your 'other' life, believe me. But perhaps it's just not feasible any longer, as your facility is obviously falling into line with so many others. A nurse who is not involved in anything other than her one, two, or three shifts is not a "merit" employee, and therefore doesn't deserve the merit pay raises. One who does nothing more than put in her time (no matter how well) , or does nothing else for the facility, is only meeting the minimum, and therefore not entitled to anything more in return.
That's the bottom line, really: if the $ increases that go with putting in the time requirement (a committee) is really important, you'll find an hour once a month, or every other month or less (they rarely ask for more!) not a big deal. If the time is of that great a concern, then you'll understand why the co-workers who DO do these committees get the raises---and you don't.
FWIW, I have spent a grand total of less than two hours on my committee work this entire year. And I'm a 'team player'!