It is not excluding you. The site I mean.
Websites, are designed/developed by web developers and programmers.
When they program and develop a site, the input screens, are programmed a certain way. Obviously, per this site, the programmers input USA postal codes. They did not, input the cities/states, postal codes or zip codes or country codes, for the entire world.
Hence, the registration "form" to join this site, will not "accept" certain formations of postal codes or country codes or zip codes. In the USA, the postal codes/zip codes, have 5 digits. Not all countries are this way. There are many different numeric codes, for each country/region etc.
But it is not being, exclusionary or saying only "Americans" can join.
Hence, the site is not saying Canadians cannot join. Nor other people from wherever.
It is all about, how this site's programmers set up the website and its components/windows/input screens/registration screens, etc. and developed it.
There are members here from all around the world.
Parents, grandparents, Dads.
Even a kid now and then, comes on this site. Kids.
And obviously, the website Owner(s), did not anticipate, that parents/grandparents from ALL over the world, might want to join this site.