They can bar people from accessing their servers if they do so by rewriting the entire slicer to be closed source and then implementing some actual security, instead of literally giving you the means of access AND the permission to use and modify it as you wish.

If I give you a template for a postcard, it doesn’t give you the right to send it with “signed, ricardobeat” at the end. These are orthogonal concerns.

They could very well enforce login for the entire app, that doesn’t require any closed source code and everyone would be worse off.

The part of the slicer connecting to their cloud IS closed source.