Kids that young don't have the long term thinking to get a system like this
I agree that this is true in a daycare setting. DS's school insitituted a system where kids that listened got an ice pop (in addition to the regular snack) and the kids did not seem to get it at all. For some reason, DS thought that the kids that were getting the ice pops were feeling sick, and he wasn't getting an ice pop because he was not sick.
At home, DS can associate a consequence with something that happened earlier in the day, but at daycare, this system has been a complete failure.