In this part of the world, and I'm sure many others, it's a case of "The devil lives in the details.".
If you wash your dishes in a pan or tub while you're camping and then dump the water in the forget it's no problem.
If you wash your dishes in the sink of an RV then pull the grey valve you're breaking the law.
Why? Because the minute it goes down the drain it becomes "sewage" and it must by law only be dumped into an "approved" system.
There's lots of such odd quirks in various jurisdictions laws. For instance, in Ontario, washing your hands after working on your car, lawn mower, etc., is in almost every instance also illegal. There's "zero tolerance" for petroleum products in the sewage system. You're supposed to use hand cleaner and wipe it off then dispose of the rag through an approved means.