Marion, when I'm cancelled last minute due to unavoidable circumstances I try to reschedule the program, if I can, for a time that is not a high-demand time in my schedule.  If this is not possible, I generally just write it off.  After all, it occasionally happens the other way around --I have to cancel last minute (or reschedule) because of illness or getting stuck in a horrendous traffic jam or having a car break down.

I've toyed with the idea of requiring a cancellation fee if I'm cancelled on short notice (less than 2 months ahead of time for prime time gigs), but to pull that policy off, there's a lot of logistics to set up first, and I'm not sure it's worth my time.  Most places that would honor the policy will also pay me anyway if they have to cancel last minute.  (Exeter House in Seattle recently did just that when my scheduled program conflicted wtih the memorial service for a resident who died unexpectedly.)

Re service to the resident vs. service to the facility, I think when you do a program for the residents as a group, you are providing the service to the facility.  They usually have activities scheduled for Wednesday afternoon and pretty much all the hours when most residents are awake, and when volunteers provide those programs, it frees up their staff/budgets for other things.  To reiterate, most states require them to provide activities, not just have their residents sitting around all day with nothing to do.

One-to-one visits are less clear-cut in that regard,  The facility does (and is probably mandated to) provide activities for room-bound residents, but there is always more of a need for these visits than is budgeted for, so if you give them room visits beyond what they've scheduled, it really is an additional service to the residents.
(When residents go back to their rooms for a nap after lunch or are put to bed after dinner, some of them stay up and awake much longer than others; when you sing for these residents, I'd say your service is essentially to individual seniors, not so much to the facility.