This does sound rather random to me, as well, and the name of the episode seems rather… uninspired.
I guess they'll explain why this hasn't been a problem before in the episode. Sure hope they would, at least. And I do kind of hope those vampire bats don't turn ponies into batponies or vampires. Well, unless they bite Twilight, and she becomes the vampire princess Twilight Sparkle who sparkles in sunlight. That could be acceptable.
But yeah, I will be reserving judgement until I see what happens.