It doesn't really. I read somewhere to do that the first time and just kept doing it. Probably better to use a driver's licence or utility bill although neither of those really proves your current location either. As I remember there is no official guidance on what to submit.
When applying for TR and ED visas in the past I just uploaded my passport main page as proof of current location. Canadian passport and was in Canada applying through evisa website. Probably not the correct way but it was approved every time (3 times).
The requirement to show a ticket out of Thailand only applies if you are entering on a visa exemption. It's not required to show anything if you have any kind of a visa, including a tourist visa (although most embassies require it in order to get the visa in the first place). The airlines normally follow the immigration rules and should not require you to show a return ticket if you have a visa.
USA and Canada are provisional members of APEC. The only benefit of using the card is to use the special lane at immigration. You do not get any visa privilege like you do if your country is a full member of APEC. Normal APEC cards have a list of countries on the back that you can enter without a visa. The US/Canada version is blank.
If you got anything more than a 30 day stamp then it was a mistake.
