If I remember correctly (and I might not) the first folks who tried to cross-server trade were interested in trading accounts (something like "60 War on Prexus for hi-lev warrior account on FV") and that's when the rule came about.

There was also a stronger feeling of "purity" about FV back then, even for just items, and that's why it's a broad rule. Plat on FV was worth a lot more than plat on other servers, too.

Personally, I don't have a problem with trading plat/items for plat/items between servers, as it has little to no impact on the game's economy or the way anyone plays.