Because it is water, not land. The Nato partners consider it international water and does not accept Canadas claims. I don't think Nato would go to war to prevent anybody sailing past Greenland in international waters either.

> The Nato partners consider it international water and does not accept Canadas claims

Or to out it another way, NATO partners don't respect Canada's sovereignty claims because the Northwest Passage is too valuable and Canada can't enforce its claim. Contrast that to Taiwan, a nation that should be independent, (and is not 'international waters') but no major country is willing to recognize it as such.

Greenland would be the same way.

And by the way, Alaska and Louisiana purchases were executed because Russia and France respectively knew they were never going to be able to hold those territories and ceded it for cash.

> Greenland would be the same way.

No, because Greenland is part of NATO and none of the member states disputes that.

We're going in circles now.

It is straightforward though: NATO is an organization for collective defense. The set of territories covered is well-defined, but does not include all territories claimed by each individual member states. For example Hawaii is not included, nor is the Falkland Islands, which is why NATO was not invoked in the Falklands War. But Greenland is included.

A response to an attack is decided on by the NATO council, not by the individual member states. So unless NATO is dissolved (which could of course happen one day but which is certainly not in the cards currently), Greenland is defended by the NATO alliance.

Taiwan is of course not a member of NATO so that example is irrelevant.