Because it’s “invisible”, nobody thinks about the substantial amount of poop that is brought into the pools on dozens upon dozens of suit-clad butts every hour. The plain fact is that
no one wipes well enough to clean away 100% of the bacteria-laden fecal traces that we all carry. (The worst of all are kids in the 4-10 range. Boys for longer than girls, ime. Have you ever seen the underwear in a 3rd grader’s laundry hamper? It. is. GROSS!

). I don’t have the exact stats handy anymore, but I remember that bathers bring,
in toto, several
tablespoons of fecal material into the average public pool during a day of heavy use. It comes in as “heinie-residue”, so we never see it or think about it— out of sight, out of mind.
It is why public pools operate at consistent chlorine levels of 2-3ppm. At that level, the chlorine can keep up with the constant introduction of small amounts of fecal bacteria, various viruses, skin contaminants & urine brought in by bathers.
In the case of a large fecal “accident”, the pool is summarily evacuated and the solids are removed. Then the pool is brought up to a chlorine level in the 3-5 ppm range. Exactly how high is proscribed by the local heath department where the pool is. Of course, pools do not follow similar procedures when a toddler’s poop stays hidden. A swim diaper with an unseen BM could actually create a bigger risk, since the poop could presumably spend a longer time in the water. But somehow people are only freaked out by free-floating turds
Me? I assume every swim diaper is loaded & that no one has cleaned their butt that day. If I can’t smell that the pool is heavily chlorinated, I don’t get in past my knees. If it seems clear that the pool has sufficient chlorine, then I get in and enjoy myself.
BTW— public pools in the US are required to keep a log of their chemistries. In most places, the logs are supposed to be available to anyone who asks to see them. How willing the pool staff are to let you see their log is a decent indicator of how strict their procedures are… basically how on top of things the staff is.