More than 1,000 workers at an Amazon warehouse in Britain will strike for four days next month, including on the typically busy Black Friday shopping day, the GMB trade union said on Tuesday.
Aside from Montana, the remaining 49 are "at will" employment states. You can be fired for any reason at any time, aside from if your employment is terminated for being part of a protected class, regardless of if you are a contractor or not. There are some jobs that have more protections, like union jobs (yay unions). There are also some employment agreements that are not at will if I believe too.
You can also leave your job at any time for any reason in an at will state.
In some countries you must submit a letter of resignation a few weeks or months in advance, otherwise you don't get resignation bonuses and your employer might even take you to court for lost productivity.
Of course, this only applies to people who are simply quitting for personal reasons.
I was recently visiting the Scandinavian office of my American company, and there was a going away party for one of the employees. When he was talking he said something along the lines of “when I resigned X months ago I didn’t know I’d be here this long” and I was a bit confused at first.
As notice periods are recognized and written into law (the law says that minimum 30 days or whatever your contract says), they can sue you for lost profit. It's basically that your contract is not up, not finished until your notice period ends, so the same enforcement applies as if you won't show up for work some day. You are still a full-time, fully recognized employee until your notice ends.
But breaking it is not really a thing here: we are used to it, and it's actually recognized as mutually beneficial. If they fire me, I still have 2 months of sure salary and if I resign, they have 2 months to look for a replacement.