The law has no privacy implications regarding FastMail: They always could, and had to, comply with legal orders for data. This is also true of Gmail and most other email providers. The primary difference here is that FastMail will only hand over your data in that scenario, but will never use your data for ad targeting or other corporate abuses of privacy.

ProtonMail and the like do more to protect you from the government, if that's your threat model, but you're going to lose out on a lot of features as well, because a lot of common expectations in email don't work with end-to-end encryption.