People seem to be missing that FTP and MQTT are generally insecure protocols. I think FTP is probably the bigger issue than MQTT. This kind of stuff is common in home IOT networks but would never pass security audit on a corporate network. Bambu is growing up, serving more corporations beyond the hobby community, and probably has been asked to beef their security up to make it easier to deploy their printers securely…
It's much more likely that Bambu is doing these moves to control their hobbyist userbase more tightly and make things easier for them, as they've seen other consumer electronics companies get away with similar moves.
It's a risky game however - this is going to land them on ban lists for West-based gov or edu installations even faster than DJI.