"Ban" usually applies only to new car sales, you can still drive your ICE for as long as it lasts. Cali is the largest North American automobile market and is the only state that can set its own air quality standards, due to its geography (ocean, mountains and sun). Usually, CARB will set a standard (like the dual catalytic converter), then other states like MA follow suit and it effectively becomes the new standard for the entire US and Canada because you can't just make cars for California. 

Anyway, a personal anecdote as to how MA makes buying a car in a different state and registering it here difficult. MA is next to New Hampshire, which has no sales tax. Thus, they need to protect themselves from everyone who wants to drive (or take commuter rail!) 45 minutes to save money on a car. While I was temporarily living in NorCal in 2015, I bought my Ford Escape and took out a small loan. I was able to pay that off by early 2017 and then had to move back to MA in late 2017, where I then needed to change my registration. First time at the RMV (DMV in any other state), they tell me that my California "Sale Certificate" is no good, and that I must surely have a "Bill of Sale" at home amongst my paperwork. I go home and find no such document. I Google it and find out that California simply uses modern language and that "Sale Certificate" = "Bill of Sale." I return to the RMV with that information, and get told that nope, "Sale Certificate" =/= "Bill of Sale" because the California sale certificate only tells the RMV when I began paying off the car loan. Uhhhhmmm-OK, why would I pay for a car that I didn't buy...? Also, this clearly states my name as "buyer" and the dealership's name as "seller" and how much I paid and on what day and location that occurred. Well, if she were to accept this document, they could get fired. OK, but WTF can I do? So, she looks at my documents and figures out a cunning plan: There was a 10-day window after which my title deed was more than 6 months old and my temporary MA insurance had not yet expired. If I come back during that time, I will get my MA registration. JFC. I finally got my MA registration during that 10-day window, but instead of the title of my car saying that I bought the car new (in Cali), it now says that I bought the car used (in MA). Thanks, RMV.