I love gardening and don't see why I should have to have netting over my garden just because someone can't leave a litter tray out for their cat.
It's not as as simple as that.
We have litter trays (inside and out for our cats) but it doesn't mean they will always use them - if a cat needs to go, do you really think s/he is going to walk 100M or so (or maybe even a few km) to use a tray?
They are animals - they will just go unless the tray is pretty convenient.