I think the person was most likely on a watch list. From the info I have gathered names are checked against no fly lists and watch lists- these lists involve terrorist related Bahaviour I belive.

I do not belive evern serious crimes will prevent you from flying over us airspace AS LONG As THEY are NOT TERRORIST RELaTEd. Essentially I think they will not not let you on if you are a threat to aviation security or have reason to belive you will hijack the plane.

I have a drug trafficking charge from 5 years ago for example and I think I should be fine, I guess I will find out this summer when I try for sure with a non stop flight to Cuba. (From Toronto)

Again if anyone has anything to add or any personal expirences it would be greatly appreciated!

@Mike95 My wife is a travel agent and I have heard at least one instance recently where a customer of hers was booked on a flight to Europe but it flew over US airspace and the passenger was not permitted to board. Can't remember if he was stopped at the airport or in advance. I suspect the name was on some kind of watch list and doubt it was for some petty crime

@Mike95 I believe US homeland receives a passenger list of any flight which crosses US airspace. I don't think you specifically need a US waiver, for example if you are flying non-stop from Toronto to Mexico. BUT, if you plane is diverted for any reason and lands at a US airport, you would be expected to produce the waiver. Not sure what would happen, perhaps they would just send you back to Canada. Ultimately it is their discretion.

do you fly internationally or just to another point in Canada? If you don't mind me asking. Thank you

I fly from a border city very often and have never had an issue like that