Why not. The French Open is the National Tennis Championship of France is it not? How prestigeous would it become if it was announced that only French citizens would be able to compete in it next year?
We already went over this, no it is not. You can't compare this to any sporting event. The French Open is one of the Grand Slam stops, and the only reason it's the "French Open" is that it happens to be in France. You never, EVER hear the winner referred to as the "French national champion."
And quite frankly, sdrawkcab is right about the staffing issue. At the 2008 Canadian Nationals, we had one HJ, and four other judges - one for juniors, one for seniors, two for masters. And even at that, one of the two for Masters was from Washington. At the 2007 Canadian Nationals, I think there was one HJ and then six other judges, a number of which were barely old enough to be Professors, and there were times where that showed. Any and all runners we get are parent volunteers for pretty much any premiere event at any level. Really, from what I've seen over the year, you could count the number of people that should be judging a Nationals-level event in this country on your hands. For the entire country. That's no knock against people who do judge here, but a much smaller player base will naturally equate to a much smaller pool of top-level judges. How many we have is a fairly proportional size to our Nationals. 5 to 7 judges works for 140 people. It's not ideal, but we make it work. How many judges do you see at US Nationals? Is it not multiple dozens? If even 100 Americans showed up for our Nationals, you would be waiting for a really, REALLY long time to get a judge if you had a problem.
I have all the respect in the world for Canada's three (is it four now?) PTOs. But no matter how good they are at PTOing, the simple fact of the matter is that we probably run our Nationals understaffed as it is, forget if we added in 100+ US players. The delays would be beyond comprehension. And keep in mind that we have, as far as I know forever, done our Nationals in hotels, not convention centers like the US does. In 2007 the space was, quite frankly, WAY too small, and this year it was pretty much exactly the right size. Our Nationals barely hit 140 people, forget the 1000+ that the US gets. If Americans began to come up, we would have to start using convention center space, and I don't think any PTO here is too interested in getting into the costs for that.
I can see why the players would maybe want it to be open. I can also see why, from a players point of view, it's a really bad idea. But as somebody that's been on the judging/running side of the coin for three years now, I can't think of a single possible good thing from the POV of the organizer/staff that would come out of having Nationals be open.