[tough]

In a more practical sense look at it this way, at a land crossing they can just toss you over. game over. thats it.

If you flew in on a plane, they have to get you deported which costs money and energy.

Also they will assume you have more capital if you come by plane than land and thus ask less questions.

[arcticbull]

Speaking from a Canadian perspective, US flights from Canadian airports (and a select few others) almost exclusively go through pre-clearance. This means you do US customs and immigration on Canadian soil before boarding your flight into the US, and the plane lands in the US as a domestic flight. It's not much different than a land border.

> If you flew in on a plane, they have to get you deported which costs money and energy.

Being denied entry can be quite summary, no proceedings, for most people. Admission is broadly at the discretion of the border officer and recourse for most people is either heavily limited or just straight up zero.

Note that for flights from non-preclearance countries, the airlines are fined if you're not admitted (averaging $3500 at the time of writing but I think closer to $10000 now). I believe they're responsible for paying your way back too. [1] In fact I think USCIS even tried to fine United after paroling a traveler during a transit that wouldn't have otherwise been permitted.

[1] https://airlines.iata.org/2016/10/13/document-verification-t...