Why did I get fired for changing my availability schedule? Isn't it kind of harsh to fire somebody for that?

Very much so, very harsh. Companies don't realize life happens. Unfortunately, companies don't think about you. It's about money.

Apparently you are unavailable to work when they need someone on the job. Whether or not it is harsh is irrelavant so if you can't be available to do the job when they need it done look for another job.

Put simply, you work for your employer. The employer does not work for the employee. Does not matter how much you need this job, but what the employer needs to keep the business open.

Even (if) they did not fire you, then odds are that your hours could have been cut to zero, since the hours you are now able to work, does not work for your employer.

Companies hire employees based on need to fill their needs,, not to schedule when you want to work, There may be room for negotiation when you interview for the job, but that's where negotiation ends

Not harsh at all. You have no right to change your schedule.

Not harsh at all. They hire employees for when they need employees. If you made an appointment for 8 am at a doctor and the doctor called and said hey I'm only available at 10 pm from now on would you look for another doctor? I'm sure most people would. They need someone who fits THEIR schedule, they aren't going to rearrange things for that person.

If you claimed you can work any schedule and then changed it to M-F 6 to 5 pm with no weekends and, no holidays. I would feel jerked around as a manager. If you want a specific schedule you should be up front about it. The new trend in food service is to utilize people on a variety of shifts and schedules on short notice.

I would try to find employment that fits better with your desired schedule or retire and try to generate income some other way.

that does sound harsh, but rnaybe they wanted to decide when you work and not let you decide