The screenshots are evidence of copyright infringement of commercial maps.

... how?

At best it's a breach of GMaps ToS by that user. And in this case the user attached a photo they took. The screenshots are just noise.

I think the google maps screenshot is poison?

But there was also the photo made by the user. I'm trying to understand the legal aspect of this. If Google felt they suffered so much of this, they should bring a (civil) lawsuit against the user that uploaded that photo. Why would OSM care in this situation?

Because OSM wants to be really really sure their data is not tainted and gets them or their users into trouble. As such, anything that suggests that you are using improper sources for editing and is noticed will get you looked at, maybe get someone to double-check your past edits, ...

And since its a community consensus thing, people will wait a few days to reintroduce a change once it has been challenged unless the challenge is obviously unreasonable. It's not like a change being delayed a few days is some unreasonable big punishment, it's just part of QA process to run. Maybe wasn't strictly necessary here, but it's a really obvious warning signal to trip.

(To make a (admittedly stretched) software analogy, if you submit a PR somewhere and show disassembly from the Windows kernel as evidence that it's a good algorithm others also use, it'll also cause some concern, and you would've been better of just showing your solution on its own)