Legally, two different Arby's locations are independent competing companies, but in practice they act as extensions of the parent company. (Similar to the "independent contractor" nonsense going on in the "sharing economy".) So we're really talking about poaching within a company and there's pretty good precedent that companies are allowed to have these kinds of internal rules.
From an ethical perspective, the fact that all Arby's locations pay the same doesn't sound that outrageous, but I don't work in fast food so maybe it's really a tragedy.
Unfortunately every company operates that way; once you accept a lowball offer (which you usually don't even know is low due to salary secrecy) you can never get out of the hole except by quitting.
