A bunch of plugins running on canvas.instructure.com are proprietary, according to their FAQ: https://github.com/instructure/canvas-lms/wiki/FAQ.

I would guess these plugins are chosen so a majority of user won't want to live without them.

It also seems these plugins "link" to canvas-lms, so keeping the proprietary would be a GPL violation if anyone except Instructure holds part of the copyright to Canvas.