The reason that WW's annoying is because it's the EARLY game he warps: He has the earliest access to the fastest move speed - and it doesn't even have to be vs the target with low hp; It can be a full HP champ on the way to the champ with low HP. For example, if WW is top, and Support is 20% HP, Mid very well be collateral damage of the support being low.
Furthermore, no other champ really punishes low HP like WW does, and many times you really can't help it. If you just killed bot, you're probably in a low HP state. All WW needs to get to you from Mid/other side of map is about a strong 10 seconds or less, and he cleans up what they left behind. You may have been planning to recall, but he just doesn't always give the time.
The biggest annoyance though is he forces recalls just by existing. You lose all lane pressure you would have had, just because he exists. You can't attempt to take a turret after a lane kill, because WW is probably going to come clean you up and he can do so at 600+ move speed when you still don't even have boots. You can't pressure a low HP opponent, because it's likely WW is going to come 2v1 you while you do. Lanes become a permanent assumption that no matter where WW is farming, he can reach you in just a few seconds. And early game, he scares teams more than an Evelynn does mid game because of this.
Now.... i generally agree that WW is preventable and avoidable by using good slows and cc to stop his chases, and heals to sustain lanes. However, sometimes I ban him anyways....... just because I don't know/trust my teams and I know WW can easily go 8/0 by level 6...... just by existing vs teams who don't live by the creed of not having a FB until 15:00. Solo Queue is why WW is dangerous. You might be the smartest player in regards to WW. Your team isn't, and eventually he becomes such a nuisance that even smart playing around him is a problem.
He's also a level 3-4 Drake taker in a meta where supports and laners no longer have surplus wards to devote to the Drake pit (Mid pink one side/ward other, Bot River bush/side bushes, top River bush and noway near Drake). Given that Infernal drake seems to spawn 70% of games first, it just seems common sense to ban the champ who can solo the drake before you can even see him do it.