It is in human nature to take a better-paying job if your current job is lousy. Of course, people make an effort to find these employees better-paying job opportunities. Who knows, you may be such a good applicant that the manager showers all the attention on you while mistreating an applicant who would have gotten the job if you had never applied. Leave it in the aisle for an employee to waste 1-2 hours putting all the merchandise back. Imagine masses of bad customers doing this 50-100 times a night.
Bad customers ruin the intended atmosphere.
They converse in a foreign language. They could be senior citizens always shopping at a store targetted at young people. They bad mouth the store loudly. Busy customers will walk out of the store to patronize another establishment. These bad customers may buy very little.