If only it were black and white. Banks are regulated by a variety of groups - the FDIC, the OCC, the Fed, and state regulators. Different regulators have different viewpoints. Then, even within the same regulatory agency, different individual regulators have different viewpoints. Meanwhile each individual bank is unique - different capitalization, risk profile, market, and so on.

There are a lot of situations where there is no hard and fast law that prohibits banking a particular customer, but the opinion of a regulator goes a long way in encouraging or discouraging any particular bank from doing so. Marijuana banking is an example of the murky waters banks must wade through. Is it legal or not? Depends on who you ask.

I'm not privy to any details, but I assume that SVB's regulators feel that the bank does not present a risk to the system by banking an exchange. SVB is the 50th biggest bank in the country or so, so they're in a different situation than a startup or community bank.

I suspect this isn't a very satisfying answer. All I can say is that in the long run we are motivated to offer banking services to every lawful business that we can ably serve.