A similar question was brought up awhile ago here, which I addressed.
"But I’d generally point you towards Hoppe’s: Property, Causality & Liability
Rothbard has offered the following “strict liability theory” encompassing both criminal and tort law.1 In every criminal or tort case, [e]vidence must be probative in demonstrating a strict causal chain of acts of invasion of person or property. Evidence must be constructed to demonstrate that aggressor A in fact initiated an overt physical act invading the person or property of victim B. (Rothbard 1997, p. 137)
What the plaintiff must prove, then, beyond a reasonable doubt is a strict causal connection between the defendant and his aggression against the plaintiff. He must prove, in short, that A actually “caused” an invasion of the person or property of B. . . . To establish guilt and liability, strict causality of aggression leading to harm must meet the rigid test of proof beyond a reasonable doubt. Hunch, conjecture, plausibility, even mere probability are not enough. . . . Statistical correlation . . . cannot establish causation. (Rothbard 1997, pp. 140–41)"
“However, he later applies a suspiciously similar principle to explain how road owners should be sued for the wrongs of the individual car owners who use their roads (p. 90)”
Rothbard never says “should”. Essentially, it is more of a “could”. He is openly acknowlegding that given what is required it is unlikely for the plaintiffs to prove that, that specific individual car emitted this much pollution etc. They are unlikely to prove it beyond a reasonable doubt. Hence: "While the situation for plaintiffs against auto emissions might seem hopeless under libertarian law there is a partial way out."
He says “would”, but what he means is - say you have a party, people are over, it gets rowdy. Someone throws a bottle at your neighbours window. The neighbours come home, they know you had a party. Who did it, they dunno, you dunno - but it was on your property. So they sue you for damages. They may not succeed in proving it was you that did it, (because you didn’t)… but the reason he mentions “partial way out” is because you then have incentive to try track that person down and pay the damages. If you can’t find the strict causal connection, you then make steps to regulate the situation the next time it happens (you hire a security guard), build a bigger fence etc… or don’t have a party. Hence: “In order to protect himself from these suits, or even from possible injunctions, the road owner would then have the economic incentive to issue anti-pollution regulations for all cars that wish to ride on his road.”
The individual / organisation doesn’t want to continue going to court, paying those costs, or the possibility of a judge ordering an injunction etc.