DJT is just following the lead of the Founding Fathers and our original constitution the Articles of Confederation which allowed for and welcomed the free admission of Canada...

Article 11 of the Articles of Confederation stated that Canada could join the United States and receive all the benefits of the union. (This was a special privilege of instant access offered to no one else.) It also stated that no other colony could join the union without the agreement of nine of the thirteen states!

Article XI. Canada acceding to this confederation, and joining in the measures of the united states, shall be admitted into, and entitled to all the advantages of this union: but no other colony shall be admitted into the same, unless such admission be agreed to by nine states.

Side note: If Canada accepted the generous invitation would they have the right to get a divorce from the voluntary compact they just entered? Or like the mafia would they be in it for life??