This isn't a good argument.

You are saying that no matter what we do there are going to be instances of something we try to prevent so why do anything at all?

Why brush your teeth? You'll have to get fillings one day anyway.

That's a silly idea that you dismiss straight away, right? It's basically the form your argument takes.

If you think rivers in America are polluted you should visit a country where the government really has no power to uphold environmental standards, it's shocking what goes on in the world.

Government regulation of business acts as a low point no-one can go past in bringing down their costs. Think about it, it's the nature of a competitive market that you have to outcompete other players to survive, so we get efficiency, everyone pushing for higher productivity and all that good stuff we want. But we also get bad stuff, things like companies dumping into rivers, but in a market where a producer has cut his disposal cost by dumping into the river his competitors have to do the same thing to survive, or they have to sell into a different market where they don't compete with that guy.

Even though maybe everyone would prefer not to dump into the river it doesn't make any difference because no-one can afford not to do it once one guy has decided he is going to, so basically you get a situation where the standard is set by who is prepared to be the most egregious asshole. That's where government's role is, they set the bottom limit on what the competitive practices can be, so now no-one is allowed to dump in the river (or use slaves or child labour or expose workers to toxic environments or whatever) and so now you have a bottom limit which collectively the society has decided 'we don't want to push each other past this point' because it does more harm than good and the bottom limit is set by (ideally) the moral sentiment of the majority of the population. Of course there are problems with the system, all the assholes are still there of course but it is something of an attempt to curb the way that factor shapes our society.