The easy answer to the question is that air mode is deemed to be a higher risk than land and marine. With land, it's not feasible to get advance information. Most people jump in their car, they want to drive over to the States to go shopping, it might be last-minute. It's not the same type of presentation of information to gain access to another country. Also you have that one on one, you have the booths set up, so the dynamic is a little bit different.
With air mode, as of course we've all experienced in the past, there can be significant security issues to the Canadian public—