This is actually really interesting to me. I just looked up the info regarding the visa required to be a foreign athlete on a team in the US. I can see why the wording was how it was in the email that was posted. It actually does make me wonder about the new Worlds rule, but in a different way than has been mentioned previously. It seems like the rule could be an attempt at leveling the playing field. For example, an international athlete couldn't obtain a p-1 visa to join the Worlds team at my daughter's gym because they have not won Worlds numerous times...one of the qualifications for the visa is joining an internationally recognized, award-winning team.
P-1A Internationally Recognized Athlete | USCIS
Eta: an athlete wouldn't need this to commute to a team, obviously. A Canadian should still be able to join a US team and attend practices without relocating.