When it's against a legally protected class and opens up the company to liability, it is rational to fire an employee for not reporting it.

Safety violations are taken that seriously at some companies, anyone observing one that doesn't report it also gets fired.

It depends on what type of culture of accountability the company wants.

I think failure to report digressions of severe enough circumstances are digressions. Eg. calling people "bitches" much to laughter in the workplace isn't acceptable. If it ever got out to the media, a glassdoor review, etc. that the company did nothing to a whole department laughing and calling people "bitches" the company would be in major, expensive shit.

Trying to take it further ("report days"?) seems like a slippery slope fallacy to me.

I agree it isn't acceptable, and I agree there should be cases where reports should be mandatory. But I'd place that line far into the criminal realm.