I can't imagine it is really a big water treatment issue since they have a different density than water and you could separate them with settling tanks and skimmers.
I dare you to tell us the cost of fitting tanks and skimmers into every sewer in California. Or every other body of water it flows into
Filtering the inputs to San Francisco Bay would be ridiculously expensive. Outlawing this plastic crap makes far more sense.
What you describe is theoretically possible, but utterly absurd in reality.
It's not a nothing issue. It's huge amount of crap dumped into waterways which acts like silt, doesn't break down, and otherwise serves to give people whiter teeth (or whatever the hell it's used for).
California has decided that's a dumb idea.