WTF? Why would you ever do that? You said you have no legal presence in Russia, why f*ck over the users? Instead of ignoring hostile stupidity you're helping it, this is mind boggling. Sorry, I can't "feel good" about your services anymore.
They were were going to be blocked anyway. So the two options were "Russian users are blocked, and the Russian government considers us law-breakers," or "Russian users are blocked, and the Russian government does not consider us law-breakers."
Even if there are no direct consequences that the Russian government can enforce on Fastmail, I can't imagine why they would want to choose option 1. Why do you think they should? What good would it do for anyone?
I.e. a choice between "russia is ineffectively blocked by us" vs "russia is effectively blocked by the russian government".
That is absolutely not the reason for this regulation existing.
Please read what you have wrote now.
Law say go spy after A and B, ESPIONAGE
Also, why should Fastmail expose itself any further, in any way as a target of Russia's corrupt dictatorship? It does suck for those Russians impacted. They should turn their ire toward the real problem though: their government.
Expose to what exactly? Sanctions, neural-gas attacks or nuclear weapons? Does Russia have any legal or technical levers which could potentially harm Fastmail business?
The effort to make that change was significantly less than the continual effort required to deal with the pressures. From a cost perspective their decision makes sense, from a customer perspective less so.
I don't know the exact ins and outs of how courts will enforce foreign judgements (I'm sure it's very case-by-case), but saying "we cut off signups from people in that country" is surely a very strong prophylactic against such a judgement being enforced.
And on a more petty level, Russia could make serious trouble for any Russian expats employed by Fastmail (or their relatives still living in Russia). Cutting off signups makes this at least marginally less likely.
Because email isn't like internet, the MXes connect directly and don't typically have intermediary relays.
If Russia decided to straight up block all fastmail MXes - Russian customers wouldn't be able to transmit anything between Russian hosted MXes and Fastmail. So their actions protect their functionality, more than their potential customers.
Making a whole effort of operating in Russia, being blocked, and trying to work around that seems like a potentially endless effort. Meanwhile you're offering a service to your customers that would be pretty spotty considering all the hassle going on.
I would also be annoyed, if my email provider provided access to my data for Russian special services.
All considered - I'd rather that they cut off Russian signups, than either of the other two options.