Remember making it "one purchase per account" would be simpler on THEIR end too, as evidenced by the people having problems just pre-ordering the fracking things. It all falls under the big ol' happy "DLC" banner that providers such as Sony, Steam, and Microsoft include in their contracts to make sure they get a slice of the pie. Sony just happens to be more "permissive" about it than microsoft.
FFXI, being one of the first ever MMOs playable on a console and having expansions that predate DLC, often gets grandfathered clauses allowing SE to take control of the game the way they see fit.
The only major difference between this and DLC for other games is that FFXIV is one of those VERY rare games that actually has cross-platform support, so the "platform lock" is a lot more jarring.