Not all cats use litter trays indoors - if anything their use of litter trays says a lot about the owners of said kitties... cats prefer to "go" outdoors, you choose to stay in a flat whatever, well that's your problem not the cats.
You lost all credibility way back cat women, true to type no loyalty, I bet if next door started offering you free Scumpy Jack and weed you'd just slope off in the night