Correct. It also used to require an upfront purchase of the game and every expansion. I'm not sure if it still does. Those who paid bought the game and should be able to continue playing it, even if Blizzard wants to shut down the servers.

It still does require a purchase, though now all expansions except the latest one are included in the base game and you can purchase the expansion with the base game at little markup over the expansion itself so the base game might as well be free.