Uh, it is an open source system.

There are closed source components to be sure, yet a lot of products that are based upon open source software are also a blend of open and proprietary. Could Kobo become more restrictive? Sure, yet so can open source developers. The people who develop open source software own the rights to that software. They may not be able to claw back the rights to existing code, but they can do so with new code. That new code can even break community contributions if they so desire.

Kobo has been decent to third party developers in the past. They know that people are modifying their devices, yet they have not introduced new restrictions on old devices and they haven't locked down their new devices. (Well, I don't know about their Android based products -- but Android is Android.) You are free to regard them with suspicion if you so desire, but that doesn't mean that they are going to do anything that is deserving of our wrath. Yes, watch them with a wary eye. Yet don't condemn them for something that they have yet to do.