This guy isn't just a little in the wrong he's a lot in the wrong.
For a moment, lets assume its okay to insult a customer in the wrong on behalf of your employer. If that was the case this guy would still be wrong -- because a tip is an optional thing.
A tip is for service, not for cooking your food. You don't tip the cooks, you tip the people providing excellent service. Except at a food truck, there aren't servers.
Agreed. I've been rather irritated lately by the proliferation of tip jars everywhere. Essentially anywhere there's a cash till there's a tip jar, now. It instantly invokes the relevant social norm, so I always feel bad not tipping, but then what precisely made someone deserve a tip because they handed me a bagel from behind the counter?