Important to note that the license they release their software under explicitly allows users to do exactly that

It’s like setting up a website with a user login, and if someone puts in “admin” in the username field without a password, the system says “sounds good” and lets you in. And then the website owners getting mad that someone hacked their system.

Blatant incompetence. I can’t believe they’re using their stupidity as an argument.

You are right, but technically speaking it would be a crime anyway. It is not that if you leave your door open then entering without permissione is not a crime.
While Bambu Labs obviously is trying to implement some sort of subscribtion model, and they are doing it in a bad faith way, for shitty as the authentication model is it is not an authorization to enter freely.