Technically correct, and that is the intended spirit of "at-will" - but that's the very thing I disagree with. I need this job, and it would represent an immense hardship personally to have to drop everything and find a new job which might not necessarily be any better.

We need more serious protections against that. At the very least, firings should always have a valid reason, and I shouldn't have to fear getting cut when I tell my employer to pound sand after they try to double my hours. There should be a cap to begin with on uncompensated hours.