Or the other way to look at it, is that there's so much bacteria, algae, rotting plant and animal matter, bird shit, fish shit, etc. in lake and pond water that a little human urine won't appreciably add to the risk of swimming in it.
They are picturing themselves drinking your shit most likely.
Historically human beings contaminating the same water they draw from has been a substantial cause of illness even if campers now are much more apt to get water from a bottle.
