How is it not? You can always threaten to quit if they don't give you a raise. The only way that doesn't work is if they can easily replace you with someone willing to work harder for less money.
And in that case that's what they're supposed to do. You would only be willing to quit if you had a better option that the person willing to do this job for less money apparently doesn't. So then they should get your job and you should go take the better job.
It's not. A company can treat its employees well and still be profitable. There is no law that says the only job of a company is to maximize profits for shareholders; in fact, this has gone to court and judged to not be the case. So, no, a company is not "supposed" to do these kinds of underhanded things. Just because they can - just because it's legal - doesn't mean they should.
How is it abuse when it's entirely voluntary? The employee is not a slave. They can walk away at any time, or threaten to in exchange for concessions.
> There is no law that says the only job of a company is to maximize profits for shareholders
Nobody is claiming that here. You're assuming the only reason companies ever do this is to screw people over, when just as often it's caused by the company's revenues declining or costs increasing, requiring the company to do more with less or go out of business.
If the company can't keep paying you the same money for the same work, the only available options are to take what they're offering or leave your job. It doesn't help anybody to take away the first option.
So spend a few weeks looking for a new job and then quit.
More importantly, you don't actually have to quit, you only have to make the credible threat of quitting. Then you get the raise because it's cheaper than finding and training a replacement.
They want to put you on call? No extra pay required. Change you to the night shift? You still have a job, that’s sufficient.
It’s an employee rights disaster.