A Kiwi friend sent me a link to your excellent collection of pictures from Kananaskis: http://www.scoop.co.nz/mason/stories/HL0206/S00169.htm My explanation...
Kananaskis is in rural Alberta. This is tough country, full of farmers, ranchers and cowboys. Real men.
When they saw these world leaders getting off the plane without a cowboy hat, they probably assumed they forgot theirs. It's like a house guest who arrives without a toothbrush--you find one for them.
Not being totally oblivious to the stature of their guests, they were given white cowboy hats. These aren't suitable for actual cowboys, they would dirty too quickly. White is more formal and dressy, appropriate for such men. At least until after labour day.
Lecturer School of Computing Science
Simon Fraser University Burnaby, BC