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.

Wouldn't it be very diluted, to the point it is no more harmful than whatever else may be in that river water?

Historically this doesn't seem to be true but obviously it was a much larger group of people polluting the water.