Spoken like a true liberal.
Government can make promises. But it doesn't have to keep them...
Well, not exactly.
It is a fundamental of republicanism that one legislature cannot bind its successors - if they could, then the People could not exercise their sovereignty by electing representatives.
These public employee "contracts" are not worth the paper they are written on, because they pretend to bind future legislatures to tax and appropriate to pay for them.
The temporary occupants of offices who negotiate these contracts are promising to pay them with money they don't control and can't raise. A mayor or Board of Education that make promises to the teachers union are promising that a freely elected legislature, in 10-30 years, will raise taxes or borrow money, and they can't promise that.