I suppose the DOT might look differently upon Country A - USA - Country B - USA with a change of carrier than USA - USA - Country B with a change of carrier (the Qantas case). I wasn't able to find any rulings on it one way or another. I agree it definitely wouldn't be considered cabotage under US law without a change of carrier (Canada could disallow it, but why?)

Anyway, if the United rep was OK with it, then so be it. As fumje suggested, I don't think one-off cases would rise to the level of an enforcement action without somebody complaining. For example, in the SPN/GUM cases, not only was Asiana blatantly advertising their offers, but I suspect Continental found out about it and tipped them off.

Incidentally, this FlyerTalk post quotes the Asiana enforcement order and describes the kind of things the Enforcement Office is looking for -- basically, "straightforward cases." This doesn't really feel straightforward, so while I think it probably is illegal, I also doubt that anyone would get in trouble for it.