Three other ideas:

What about French laws related to health (are they like NYC and trying to enforce health policy through food ingredient restrictions?) Could that be something they would not want getting out by accident?

What about French labor or immigration law? Could there be some violation that they didn't want documentary evidence of?

McDonald's as a corporation protects the intellectual property of their operations with great effort (but usually in the court of law--as when a manager might try to start their own burger place using the official playbook). Not sure what competitive advantage he could get from a few minutes in the store, though.

But all of these really hinge on the employees thinking that the wearer a) wasn't a government official or inspector (confrontation would just bring more heat and/or b) that the wearer was of lower or weaker status. Mall cop enforcing a poorly understood "no pictures of the mall" policy over-zealously seems like the most likely case--maybe rooted in anti-terrorism paranoia.

(Or else maybe the guy made the mistake of ordering a Royale with Cheese and these employees were not Tarantino fans...)