The barrier protects citizens. Using magic could hurt citizens. No barrier, no magic from the good guys unless they want to risk causing collateral damage to more than the scenery. That'd be my guess anyway. Sort of a self-imposed limitation on the workshop witches because they actually give a crap about what happens around them?
Alternatively it could be similar to things where someone has to have something like a contract with the "land" for certain things to work there. Couple examples of that come to mind from other things now that I think about it. If the chairwoman is the contract holder for this land, and she's out of action, maybe even the tower witches can't properly do anything at normal power level in the area the contract handles.
Or both? Neither? I don't know, I'm half asleep and tacking stuff on to a post I already made a few minutes ago.