If they're not paying for hours you actually worked that is wage theft. But if they're merely adjusting your hours to make you a part timer so they don't have to cover your benefits, that is sadly totally legal.
Would you pay someone who walked out mid-shift? They are required to pay you up to that day. Not for the time you walked out. And they might not have to pay you for two weeks prior since you failed to give notice.
No, it isn't illegal for them to fire you for walking out.
It would be illegal if they fail to pay you the wages they owe you. Do you have proof of the hours you've worked (eg. timesheets) and the wages they've paid you (eg. pay slips)?