Imagine a garbage company. The government pays them so they can buy land where they dump their garbage. Obviously a subsidy.
Now, let’s say the government buys the land themselves then gives it to the company for dumping. No money changes hands but this is still pretty clearly a subsidy.
Instead of giving the company land, the government retains ownership, but lets the company dump there for free. Still a pretty clear subsidy.
Instead of buying the land, the government just takes it. Now no money is involved at all, but it’s still a subsidy.
Instead of taking the land, the government just declares that it’s legal for the garbage company to dump trash on other people’s land, and the owners just have to deal with it. This is quite different in the details from the original subsidy, but the overall effect is essentially the same.
Polluters are in a situation that’s exactly like this last scenario. They get to dump their trash on everyone’s property and don’t have to pay for the privilege. They’re being subsidized in an amount equal to whatever payment it would take to get everyone to willingly accept this trash.