My mom accidentally did that once. She left an apology not and insurance information on the windshield.
Though I guess it really depends on who owns the car...

Depends on what you define by scratch. T-boning a car at full throttle is NOT a scratch, Ry

Huh, that's funny. Somebody backed into my mum's car the other day, banging up the front a fair bit, and drove off without leaving any details...

... Ry...? Do you have something to say...?

You forgot the 'blame it on someone else' option.

I don't drive. But I once bumped a bit into a car when I opened the door. And I tried to look and saw no scratch on the other car. And the woman in the next car got really angry and I tried to blame it on my mother, when I clearly told her that it was me and not her. But that lady still insisted that it was my mom and pointed fingers at her and lied how she saw my mother doing it and all...
And after all this, there was no mark or anything from it.
Apparently she thought we was the most "rudest" people she had ever met in her life and all too, gees apparently apologizing is "rude" now a days.

It depends on the situation. Last week, a very crappy van was parked next to me, and because the spaces are close, I had to watch out not to scratch their car more. But since it was already scratched to hell, even if I did scratch it, I probably would've driven off. I would've felt bad about it, but hey, it was already scratched to hell. What's one more?

And your last choice works too: They shouldn't park in tight spaces if they didn't want to scratch it more. If you want to protect your paint, you better park in the middle of nowhere or get a covering.