Nothing _prevents_ it, but

1. Fastmail doesn't have the same reputation as Google of banning accounts with little to no recourse (presumably because their customers pay, thus significantly increasing the cost for spammers to abuse their platform).

2. Even if Fastmail bans me or the company goes under, I still will own the underlying domain and can move somewhere else.

Is 2. not true for GSuite regardless? It's one of the reasons I find the risk profile acceptable despite Google's ability to ban me.

I believe GSuite supports either @gmail.com or custom domains, so it can be true for it.

Edit: my bad, this is apparently untrue.

G Suite doesn't support Gmail, but I think one of the bigger problems is that other aspects of your online profile (YouTube, Play Store) are tightly coupled to your Google account.

Your account can be banned, but unlike Google you can get in contact with a human being and get an explanation or sorted the issue out if it's a mistake. With Google you never known when an algorithm will ban your account, and if/when that happens, there's absolutely NO way to get it back, your account is gone for good.