Cats do have right to roam but owners have a responsibility to prevent damage to other peoples property as a result of their pet roaming around. So the owner is responsible and accountable for damage such as scratches to neighbours cars or technically fouling in the garden. If proof of the acts are brought to the attention of the relevant authority, they will assess the damage and either request that the owner take reasonable steps to prevent it and pay for damages or ignore the complaint on grounds that the owner cannot be reasonable held responsible given the evidence or circumstance.
In reality this often means, unless you have a picture or video of a cat n your car with scratches where it sits - nothing will happen. There is little an owner can do about cat pooping somewhere else without cruelly keeping a cat indoors or having them walk round other peoples property and digging up their garden regularly and that is if you can prove that the cat did the dirty.
I will admit that i have owned my current cat for nearly a decade, he has never pooped in my garden and i very, very rarely have to clean the letterbox litterbox. So if you live anywhere near me, i am very sorry for all the cat poop - i honestly have no idea where he is pooping.