Although I am not an expert in this field, I believe that a law or an executive order could be passed requiring that if an external entity (Fastmail) wishes to engage in business with a US entity (server provider), end-to-end encryption must be enabled with a backdoor.
Additionally, when an individual pays Fastmail, they are essentially making payment to the server provider located in the United States.
Mistakes have been made. Fastmail is hosted in the US
Slowly moving services away from the US. Went with Fastmail, which I used before. Was reading their privacy page only to find out they're hosted in the US, even though they're an Australian service.
https://www.fastmail.help/hc/en-us/articles/1500000280221-How-Fastmail-provides-a-secure-service