He was drunk! The bar wasn’t opened yet, and the ceremony was still one hour away. Where was he getting his alcohol from? He kept retrieving beers from the back of his truck.
What’s the problem here? Yes, he may be a distraction during the ceremony and reception, but also he’s a liability. The venue can get in big trouble if he drives drunk and kills someone; he may lie and say the venue served him and didn’t cut him off.
The bar tender noticed this man was drunk, and went out to confront him. “You need to stop, this venue could face financial ruin for your actions. You cannot be here if you’re going to act in this manner”.
The venue and bartender were absolutely right to do this. I suggest maybe putting some groomsmen or uncles in charge of keeping the peace if you know some guests may get out of hand.