The question is: what ought to be done about this? I don't think a blanket ban on other than "minor car repairs" is it, but I also don't think the type of person you're talking about here is going to be too fazed by fines and inspectors.
I think if you banned all automotive work at home (including oil changes) that would reduce the amount of environmental damage like this. It's far easier to catch someone changing oil than dumping it. However there are obvious socioeconomic reasons why I don't think this is something that should be done.
I'm not proposing a solution, I'm just pointing out we shouldn't ignore the environment here. Or expect poor people to care about the environment because they very much do not. I know the people that do this and if I challenged them on it they would say "the government pours oil on their own gravel roads to keep the dust down anyways" or "if the government cared about the environment they wouldn't have killed Stanley Meyer" or some other claim to point out perceived hypocrisy.
I'm completely against this. You're expecting poor people to pay for things they can do themselves if they're willing just because a few people dump oil in their weeds at night. Your freon argument doesn't sway me either. Wrecks cause freon dumps all the time. Your neighbor pulling freon out of one car isn't causing that much environment damage.
Also: if you're concerned about it, report them.
Sure, and if you banned all alcohol consumption, then that will reduce the amount of kids damaged by alcohol.[1]
Still doesn't mean that banning is an idea worthy of consideration.
[1] The negative effects of alcohol on a population is far worse than many other things that are banned, and yet alcohol is not. Many things would get immediately unbanned if the bar was "needs to be more damaging than alcohol"