It's a territory thing.
Cats bury their turds if they do them somewhere they consider to be outside their territory, so as to avoid potentially dangerous confrontation. This cat has obviously established that your garden is his territory as nothing else is shitting there so brazenly.

Solution: send it a message by shitting in your garden yourself.
(eViLegion Chief Commissioner of the Scottish Lunacy Board, Thu 14 Aug 2014, 17:24, Reply)