Copa is a stickler for this, even for final destination of countries that do not require return or onward ticket proof. Such as Uruguay, where I've lived for several years and have legal residency. I often buy either a one-way US-to-Uruguay or a round-trip originating and thus returning to Uruguay, and Copa always wants to see my Uruguayan cédula - including reading the back to verify it says "residente legal".

Uruguay doesn't require any such thing, according to both Timatic and their immigrations website. In fact, you can only apply for Uruguay legal residency from within Uruguay (unlike most countries.) Which means for anybody from countries that don't need visas to visit Uruguay, you would arrive on a 1-way flight, getting a 90-day once-renewable tourist permit stamp in your passport, then within 180 days of arrival, applying for change of status from tourist to resident, to start the process.

Nonetheless, Copa always demands the return/onward ticket or proof of residency.