The standard says “don’t resend,” it doesn’t say “assume the worst and begin removing user from all systems.” That was the mailing list software’s decision.
You generally avoid sending to known bad addresses or your reputation will be destroyed very quickly. The 550 response is (read: was) a clear "you fucked up, this user doesn't exist" prior to this.
I saw someone on Reddit say his SES was suspended for sending tons of bounced emails in a short period of time - it's taken very seriously by ESPs.
