> Speaking from experience, there are plenty of poor people that are at least as aloof, entitled and rude to service staff as rich people, often times even worse.

Yup. One of my brothers was a waiter for many years in a town where it was easy to tell the lower classes from the upper; it wasn't just common for him to receive no or little tip when he waited on members of the lower classes, it was the rule. In years, I don't believe he could count on one hand the number of times he received more than a dollar (on tabs of tens or hundreds) or a piece of mucus smeared on the receipt (yes, you read that right).

People are people, and many people are jerks. Money or lack of money just gives them different outlets to be jerks.