They can mistreat the employees that are trying to help them.
The other day I was out with family and a customer at the end of their meal starts yelling at the waitress. The waitress is trying to get the customer to calm down. On the way to go get the manager, she sees the manager approaching the area, and I can tell the waitress is trying to get control of herself.
As she approached the table I told her that she's 'doing a great job, the food is delicious, and relax and to look into my eyes and you were getting sleepy'. The waitress's smile lit up her face. That's when she told me that the customer just left without paying for their food and the manager is going to take that from her check, the bill of all the people that was at the table!
When paying for my meal I left her a tip and an extra tip, and so did everyone at the table that was with me. When I left her the extra tip I thought to myself 'this could have been my daughter and I would really want someone to encourage her not to quit, not to give up'. This probably was her first job, and having to pay for someone's meal, or should I say several people's meal, because you were trying to do your job simply was not right or fair.
In case you didn't know, so many restaurants, when someone walks out without paying the bill, they just don't write it off, they take it from the waitress who served you. The one who provided service with a smile.
So be nice, pay your bill and tip the waitress or waiter.