Bambu Lab has made plenty of mistakes, but I don't think this is one of them. And I'm a big supporter of open-source software.

Their cloud infrastructure obviously has real costs associate with running it, and I don't understand why any software other than their own should be entitled to use those resources.

If you buy something and then significantly modify it, you generally tend to void the warranty - and that's not because companies are just greedy; there are real limitations when it comes to a company's ability to support the endless ways a product could be modified.

Publishing something as open-source does not imply that you must operate an optional-but-complementary service at a loss for charity.