I see what you mean granjura when you say people should be able to leave their gardens overgrown if they wish to. But to some extent when you're living in close proximity to other people, I think you should have some consideration for them.
I think it would be perfectly reasonable to have a fairly wild, "wildlife garden" - these can look quite attractive or at least reasonably inoffensive. But to have a garden that is completely overgrown with weeds and which is visually very unappealing I think is inconsiderate - even if the garden didn't affect the neighbours' fences, gutters, etc., etc.,
I would hate to live next door to a garden like that and I'm sure some other people would feel the same - thus making it more difficult for the person next door to sell.