Your store may not have been a training issue, but the one I worked for specifically told us it was because they did not want to dedicate the resources to training us for even other US State IDs let alone international. I do agree it also serves as a safety net as it limits the IDs one must learn and recognize, but I only stated what was true for the store I worked for.
You'd be hard pressed to guess my age. I'm 31. I was mistaken for a 12 year old today (a little more extreme than usual, but I'm rarely identified as older than late teens). Person at the restaurant (not Disney; local buffet place) was almost insistent on giving me the child's meal price until I convinced them I was an adult. I have a friend who is only three years older than me and has the opposite issue. He has been mistaken for my father a number of times (to the point of being told his "daughter" needed to sign the child form). I've always found age guessing to be a flawed process because of this experience. I much prefer IDing everyone. A local concert venue operates under this policy. But it's not law. ID checking itself is to protect a business from unintentionally breaking the law.
Note that "looking under 30" was specific to the store I worked for. There are some stores that card if one looks under 45. Others, like the concert venue I mentioned, card regardless of age appearance.
I don't drink, but if I did, then I would rather someone refuse me than risk their job.