The easiest, safest explanation would be the presence is for solo use, and practical for easy clean up.
Seriously though foreign objects finding there way into an msd system can only happen on a boat that has passengers, and a part of the predeparture briefing should mention it. Of course this solution isn’t failsafe, just saying…
This kinda reminds of a time I went to work on a utility boat years ago. First thing I was told when I got on board was no toilet paper in the toilet, put it in the trashcan. I was like ‘what??’. I was then told the shower will back up if you flush toilet paper.
If a boats plumbing problem is that far gone it may be time to replumb a few things instead of making filling trash bags up with used toilet paper standard practice. Wtf?