I was lurking here last year during the racial justice protests, and before that during the James Damore controversy. I still saw casually racist/sexist posts get upvoted fairly often. A discriminatory argument can be stated well and seem reasonable on its face, but still be discriminatory. It's the kind that moderators here are too lenient against and won't really want to delete as long as there isn't a pattern of it, because it could just be an well-meaning person suffering an error in judgement. I don't know how our world is supposed to move past that, or if it's even possible. I would have probably been a little less disappointed in the state of humanity if some of those posts had been flagged, because there were a lot of them across the whole internet.

Is being racist against the law? Why would someone tell law enforcement?

Racism can be considered hate speech, which is illegal in places with no right to free speech such as most countries in Europe.

For example: https://www.report-it.org.uk/reporting_internet_hate_crime

Incitement to racial hatred, done publicly, can be held illegal if it crosses certain thresholds. I don’t think this means Europeans don’t have free speech.

Consider both:

https://en.wikipedia.org/wiki/United_States_free_speech_exce...

https://en.wikipedia.org/wiki/Privacy_laws_of_the_United_Sta...

I don't know why you're getting downvoted when you're correct here.