While I can't speak for Grak, the vast majority of workers in the US have no contract, and can be ordered to work any number of hours that the employer wants, when they want, and can be terminated for any reason, or none.
That sounds ever so slightly illegal.
It's called "at-will employment", and most places have a line in their handbook (which you are required to sign as a condition of employment) that reads something along the lines of "I acknowledge that I have signed no contract with [company] and thus my employment can be ended by either party at any time for any reason." The laws enabling it were intended for temp agencies, which as originally envisioned were for topping off your workforce at-need in the event you found yourself temporarily but indefinately understaffed (if, for example, you got a very large order that needed filled, you could bring in a lot of people quickly just to work on that order) without going through the full hiring process. Nowadays, pretty much all companies in the blue-collar and no-collar market staff entirely through such services, either by the "89-day temp*" method or by actually hiring them after 90 days.
@MaximumZero Right-to-work is an entirely different kettle of fish, in that it applies to companies that have a union contract.
*I don't know if it is law or simply part of a generic contract, but temp service employees have to be hired permanently after 90 days at a location. Thus, it is common for employers to release a given temp on the 89th day.