I think the assumption is that buy-to-play MMOs tend to be less microservice-based or 'cloud native' than subscription ones, such that they are more likely to already feature a monolithic server application. They're probably thinking games like ARK: SE, and DayZ, rather than Guild Wars. In reality, some sub-based MMOs have monolithic servers (e.g. Mabinogi), and some buy-to-own MMOs have distributed architectures.

This was probably also an easier sell to politicians, by saying, "hey, they said they sold the whole game for that price, so why can they not deliver the whole game, server included?" With a subscription, it becomes harder from a business 'rights' perspective to argue that a player who paid for e.g. 1 month of a subscription immediately before the game is retired should be allowed to then own and operate the full game indefinitely, and then becomes a sort of, "how long paying the sub is long enough to 'own' the game?" debate. This is especially important because it could impact a lot of non-game software as well, so politicians are much more likely to quash this out of fear of backlash. So they may just be picking their battles.

WRT market impact, I am sure the shittier companies would use the exemption as a loophole, and just make all their multiplayer games subscription-based. I doubt it will encourage more buy-to-own MMOs in the future as well, but I think SKG cares more about the extant software people paid for already, than the market impact.