On the grounds that a big and valuable chunk of territory that is currently being shared shows signs of being unilaterally fenced off. I’m not suggesting that Canada has a better claim, but it’s important for procedures to be followed.

Edit: I wanted to get my wording right, so I went back to the article:

The legality of all this is a bit hazy, Treadwell explains in a post for the Wilson Center. To make the definition official, the US has to submit data and reports to the United Nations Convention on Law of the Sea (UNCLOS). However, the US has not ratified UNCLOS due to complex political disagreements (the agreement has been ratified by 168 states and the European Union).

This leaves some uncertainty around how the proposition will be accepted under international law.