My understanding is that in an “at will” state, employees can be terminated for any reason other than being a member of a protected class. Can an employer institute a policy that pretty much everyone will violate sooner or later, and then fire some people for violating that rule, but not others?
Very general not-state-specific answer:
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Employers aren’t required to enforce workplace rules fairly or even-handedly.
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Employers can’t use the rules as a pretext for a termination that would otherwise be illegal (e.g., discrimination, violation of public policy, retaliation for making certain kinds of claims).
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Employers often lose unemployment claims based on work rules that go unenforced, pretext or not. If the employee can show that several others broke the same rule and that the employer knew of the violation and did nothingh, the employee can often convince unemployment authorities that the rule wasn’t a good reason for the termination.
This doesn’t answer your question, but I once was fired at a very large worldwide company for using e-mail for personal use. I asked them if everyone who did so was being fired. I was told no, just you. Could I have fought this and won? Who knows.
SOME states have rules that preclude termination in violation of “public policy.” California is (or was when I lived there) one of those states. Sometime next week, when I get a chance, I’ll look up the statute involved. I’m leaving for my 30th reunion in California tomorrow evening, so I won’t have a chance til then. 
My experience in HR is consistent with what Gfactor said. Good HR practice is to be consistent, but not every failure to be consistent has legal consequences.