Because they don't have a summoner spell that gives you the ability to teleport either, right?
There's nothing wrong with this sort of a thing as long as it's done in good measure, and as I assume you're talking about Rek'Sai's ult, I believe hers is as well. It doesn't do any damage, it doesn't have any sort of special effects outside of a brief MS boost, it just gets her from one place to another. And in a way that a player knows it could happen; if you're alone, at low health, you probably shouldn't stand near one of her tunnels. She can't pop out anywhere, you know the places she's going to come out from just like you know when someone is TPing to a minion or a turret, and just like you know where Twisted Fate is about to ult so you can point all your CC at him.
This just gives new avenues for playstyles, engaging fights, or strategy when pushing. That's it. If you don't like mobile champions, get first pick and ban them. If mobility was really such an issue, champions who had high mobility would rule the rift, but they don't. They're strong when played well, just like anyone else, but easy to whiff because of it.
