The McDonalds website says “For Technical Support, please call 1-877-MCD-WIFI (1-877-623-9434)”.
That’s an AT&T number.
The person who answered said “sure, no problem – you can sit in the parking lot and use it” when asked if a non-customer could use the signal.
Calling 1-800-244-6227 (the McDonalds toll-free “contact us” number) produced this answer: “yes, people outside the restaurant pick up and use the signal”. When asked explicitly if that was OK, the response was “uhhh – maybe ask that specific franchise”.
Here are factors I can see playing a role:
1) Does it cost McD extra money?
2) Does it degrade service for other customers?
3) Are they happy to have you use it anyway, for goodwill (and the fact that when you logon they advertise via the splash screen)?
4) If you occasionally pop in and buy a soda or coffee it may be worth it for them.
5) If you potentially may pop in and buy a soda or coffee it may be worth it for them.