I operate an auto detailing shop. As part of that I've done some research and spoken with my local city (100k+ pop.) officials about this. It's actually quite logical.

First, there's a distinction between sewer vs. stormwater. Sewer lines go to a treatment facility that's built specifically to take all the bad stuff out of the water before flushing that treated water into your local streams. Washing your car into a sewer drain, all good.

Stormwater drains shuttle water directly into your streams.

Stormwater drainage is purpose-built to handle the overflow rain during storms, and only that. In fact, the first goal of stormwater management is to not drain it at all! You want the stormwater to flow through your local ecosystem naturally, generally as groundwater. Nonetheless, storms conspire to drench our non-porous surfaces (asphalt, concrete, etc.) at a rate or duration above the designed for drainage of the system, resulting in overflow. Overflow leads to things like flooding or public safety hazards for cars driving on undrained roads, so a secondary goal of stormwater management becomes shuttling excess water out of the local ecosystem.

What's all this have to do with washing the mud off your car? Well, the first goal of stormwater management is to keep it in your local ecosystem. So, if you can ensure the runoff from washing your car goes into your grass or a specifically designed catch basin, then you're all good. But, if you wash it off into the stormwater drain, well then you're using that drain for a purpose it wasn't built to serve. Your water is neither excess nor should it bypass your local ecosystem. As far-fetched as it may sound, that mud may have local nutrients, pollen, chemicals, etc. that could serve your local ecosystem, and by bypassing that you are disrupting your ecosystem's natural cycles.

A note to the astute reader that says well, we already disrupt our ecosystems with other human activities. Yes, you are correct. That doesn't mean that we can't nor shouldn't take actions to minimize or eliminate further disruptions when they are within our sphere of control. We must strive to find a balance in ecological systems.

Thank you for this very detailed write up, this actually clears up a lot.

I appreciate you taking time to explain all of this, it is pretty baffling otherwise

There's also an enforcement aspect, too - it becomes significantly harder for police to determine the difference between "I was hosing it down" and "I was hosing it down and washing it with soap" so they just ban all of it.

My understanding is that it’s not just about the soap. But also to restrict the amount of oil, gas and salt getting washed down the storm drains.

Places like Vancouver use street cleaning machines in the spring to sweep up any salt on the streets.

I’m skeptical of the ‘big clean’ lobby being able to buy this law, I could be wrong.

It’s usually not just mud on your car and all the other stuff is also not good to let into the water untreated either

And you can’t feasibly regulate individuals to only put “mud” into the sewer system, but you can regulate and inspect a car wash.