See paragraph 2 of this and you will see that we must stand to disagree.
What I learned in school agrees with what this article says, from the National Review.
http://bench.nationalreview.com/post/?q=MzJmZDEyNWFiOGI4Yzk3ZWI2NDVlMGE3OGYzOTUzY2M=
Here's the actual treaty in which the US got the rights to the Canal Zone:
Key section is "which the United States would possess and exercise if it were the sovereign of the territory within which said lands and waters are located".
The US paid for the "use, occupation and control" of the territory, it did not "become" US territory.