My understanding is that, yes, the pharmacy schools DO know where you are holding seats. I originally sent in a acceptance and deposit to Western. Then, when I got accepted to UOP I was told that I HAD to withdraw my acceptance from any other schools that I was accepted. I think they gave me 2 weeks (which coincided with my deposit due date I think). I was basically told that if I still had seats being held with any other schools that they would cancel my spot.
When I was accepted to UCSF however, I'm pretty sure that I was never told anything about dropping other schools.
Different schools, different priorities....but I can see how it would be annoying to a school to have someone occupying 2 or 3 spots w/ different schools....only to drop them at the last minute. This causes not only a big headache for the school as they try to fill a last minute spot, but also for the person who only has days to get ready for the new program...or worse, has to start AFTER orientation has already begun! We had 2 of those in the fall and it wasn't fun for them.
Edit: Of course...as mentioned before, this is probably only for PharmCAS schools.