McDonald's was beyond a shadow of a doubt liable, if the facts presented in this story are true.
You do not allow people to just "hang out" at a retail place of business. You do not allow shirtless people into your place of business and you sure as heck to not allow hundreds of people to hang out shirtless in your parking lot. I am surprised this McDonalds was not fined and shutdown by local government.
If I saw anything remotely close to what was going on there at my little beer joint I'd immediately shut the place down and tell everyone to go home. (Which I have been known to do on rare occasions) Customer safety is a primary responsibility of every business owner.
I wrote that before I read the whole story. Looks like the manager was at fault.