I think that lots of cat owners wilfully ignore the problem of cats crapping over their neighbour's gardens.
OP I do see your point but I can't see how cat owners can be made to be responsible because the fact that they get a cat in the first place shows that they are not really that bothered about the impact on their neighbours. There are a small minority who do care and take steps to control the problem but they really are a minority.
Cat owners should be required to own a litter tray. I know a woman who had a cat who used her litter tray. This woman decided she was tired of dealing with cat litter and didn't like the smell so she chucked the tray out. This caused the cat to crap on her neighbour's garden. This neighbour now has to pick up the cat shit and as a result has refused to feed the cat when the woman goes away and basically relations are now very frosty. All because the cat owner did not want to deal with her own cat's shit and thought it would be ok for her neighbour to endure this instead.
Personally, I live in an area where houses have been tightly packed in by greedy developers. Between my neighbours they own 4 cats, who I saw crapping in my garden. I have paid for cat spikes along my fences, blocked up any holes under the fences, put extra planks on my garden gate bottom and top etc to try and keep these fucking bastards from shitting on the small area my kids can play in. I don't dare ever grow any vegetables because of cat shit. Prior to this, I was shovelling up a lot of cat shit every week and taking it to a ditch in a wheelbarrow. They still ruin my front garden flower beds so nobody in my family ever goes in the front garden. Fortunately it is tiny so not too much of a loss.
At the very least, cat owners should face an annual charge. What about the gloves I have to buy and then throw away to pick up their pets' turds, the plastic bags etc.
There are a lot of cat owners on this thread suggesting that nothing can be done. Yes, plenty can be done. Like not getting a cat or any more cats because it is antisocial to get a mobile shitting machine which your neighbours have to clear up after. And which generally doesn't even like you in the first place - if the cat finds someone with better food/more comfortable then it will dump you and live with them instead.