Basically ZOS did whatever was simplest for them (and simplifying the purchase process).
If you buy something while logged into the game it only applies to the server you're logged into, because otherwise it would require an extra step (behind the scenes) to send the data to the other server so it knows what you've got.
There probably is a way to fix that, so data on in-game purchases is shared between both servers, but I don't know what it would involve and I assume ZOS don't think there's enough demand to justify the time and effort required.