This is better than no work...
If you can't commit to random hours, then don't take a ZHC?
From what I've been reading, there is a pool of employees on ZHC, and that is for when one person says ' I can't make those hours do to XYZ' So they offer it to someone else in the pool.
I doubt they get rid of you for turning down one piece of work.
If you're in a situation where you need the money, you aren't going to turn down work, you will find other ways to allow you not to be busy - I.e. baby sitter, find someone to do a school run etc.
Chances are most situations they give you a few days notice of your hours...so it gives you time to plan around it...
Again, not seeing the issue, any work is better than no work.
If you have other commitments, don't take ZHC and go find a part time job that suits yours hours...