Hi Sneaks, yes that's the thing. If someone is asked to work on, say 3 projects, when they are realistically meant to be have adequate time for only 1 project, he/she is bound to under-perform, isn't it? And then the PI can take that as an excuse to get rid of them. That's why I said it sounds dodgy.
As a matter of fact, postdocs receive fixed-terms contracts to do specific projects. So isn't it illegal to ask them to do more than that?