This is true. But it may not cost them anything to keep the stinkers open. Or not enough to matter, anyways. Once you're a certain size, shrinking down actually RAISES effective costs. Weird, but true. Now, they MAY do other things with those servers. Or they may make them... Idle. Or do something else entirely. But keeping them open likely has no, or only incidental, costs. While closing them... The screams of the pessimistic and the scandal seeking will rise from the rooftops. So, its a matter of if the PR hit is worth it.
Also, remember, that for some strange reason, some people LIKE deserted servers. I know of some raiding guilds that prefer them, so that they have less likelihood of ques, a server crash, or other shenanigans interrupting their raids. Once paid server transfers happen, the likelihood of them being utilized by the player-base for SOMETHING goes WAY up.