Dude, it all boils down to "How much risk are you willing to take?"
I know if I invest thousands of $$$ into a tank, I want my risk-factor to be as MINIMAL as possible - don't we ALREADY have enough things that pose a threat to our tanks as it is? Why make more of them?
Why not just make life easier?
I'm not positive on the physics of it, but what if soap residue is like Mercury or PCBs and they build up in your tank over time? That would be very very very VERY not good
But I have thought about people bathing in lakes or rivers, I agree it could have adverse effects or none at all, depending on dilution levels