That isn't a very good analogy, but still, why not? If Ford's parts are better, why wouldn't a Nissan owner be allowed to use them? Yah, Nissan might revoke the warranty because of using third party parts, but Ford might cover that as well.

Better analogy: how would you feel if you're not allowed to use third-party ink cartridges for your printer?

Best analogy: how would you feel if you're not allowed to install another OS on your own pc?

Jolla not supporting the N9 is one thing. But I'd even pay for it! They might not get their hands on the necessary binary blobs for the drivers (ugh, damn those chipmakers! I've bought the damn thing, didn't I?) but citing legal reasons sounds weird to me. And for the sake of interoperability quite a lot is allowed in the EU.

Personally I consider tablets, phones, routers and even tv's as "general purpose computing devices", and would probably welcome some ruling that this kind of devices may not hinder the owner to install and use software on them as he sees fit. Yeah there are some gray areas and more issues with that, but it should be the rule of thumb.