I realized this might actually be intentional after posting seeing how the game has some other 'balance decisions' in other areas that I strongly disagree with. If this isn't a bug, it should be changed. Not being able to equip light plate armor (even if it's heavier, it's still 'light'!) will really gimp any non-warrior/defender heroes (the only classes that can wear plate) for the faction with this light plate trait. As Gilden, you'll end up with heroes running around in leather for the whole game even if you're playing the faction that's supposed to be about armor. Light plate already has penalties to initiative and costs more, it doesn't need this further disadvantage.

Sorry, this wasn't meant to be a balance discussion; I genuine thought it was a bug when I posted.

Just for one to be aware, that one may purchase armor (of any kind) from allies, and also drops happen of any kind. So these people with light plate at home may purchase the Chainmail in another land (a technique of also buying horses for heroes when you don't have the tech (but they do).