Interesting question, but the fact the activity is legal under state law is simply irrelevant. While the debate may rage within California, the
DEA (and probably the rest of the country) doesn't take it too seriously. Even if a bank chose to rely on the state law as its shield, California's use of "local" standards would dictate that you would need an intense level of knowledge of your customer's operations and local law to be certain their operations were in compliance with state law.
Regardless, federal law enforcement could definitely go after a bank that accepted these accounts and did not classify the activity as suspicious based on federal law; i.e. did not file SARs. I can also imagine the Feds making a "poster child" out of a bank saying it was complicit in the violations of federal law.
Should any bank want to open an account for a business when it knows it will be filing SARs on the account activity in perpetuity?