The itch from just one flea bite can be maddening to both pets and humans. Been there - done that! Then of course there is the issue of how sick it can make some cats or dogs.
If they don't treat for the fleas, then their house will soon be overrun with them. Not a place that I would want to visit, that's for sure. And I can't believe that someone actually thinks that there are specific species of fleas - one for cats and one for dogs, and that the 2 don't mix. OMG...:rolleyes::rolleyes::rolleyes: Fleas aren't fussy - cat/dog/human - as long as it has a warm body, then that's all that matters to them.
You did the right thing. Hopefully the cats can be rehomed since it seems like no one seems to care about their well being.:mad: To me what you described is abuse as far as I'm concerned and they shouldn't even own any animals.