I'm a person.
I do not "treat fast food employees like crap."
If you are basing your question on personal experience, I presume you are referring only to the small population or poor examples of humanity with whom you've had to interact. Rather than remember them, remember those who did not "treat fast food employees like crap".
If you are basing your question on viral internet posts, please remember these are posted to get a rise out of the viewer. Posting a nice interaction would not get views.
Not everyone "treats fast food employees like crap". Only poor examples of people looking to get whatever frustration/aggression they have out on the next convenient thing. Sometimes it is their dog or child or significant other, but in what you may have observed, it is the hard-working employee (whether fast-food, retail, airline attendant, auto-repair person, or whatever) that did something not quite right that sets the low-life into a rage.
Pretty much anyone who works with or for others will experience the same with customers or even co-workers - heck, I've been in the telecom industry for 30+ years and it still happens - not often, but it does happen. You just never know what sets another person off... Sometimes you only get yelled at... other times Law Enforcement needs to get involved... and every step in between these extremes.