I believe US citizens would be allowed to travel by plane on their passports and those from visa waiver countries with the ETA.To keisul86 I crossed the border by land from the US in a private vehicle driven by a friend of mine & I used the ECOPR with my passport & no problem. You can only travel by land either by a private vehicle or walking across the border. You are not allowed to cross by any commercial vehicle including air or bus.
(I presume you got ecopr within Canada and that's why you weren't issued a visa.)