Why not reward them? These people still do it as a race, to try and get a world first. What's the problem with having some official recognition of this?
Because having official recognition means that more people will do it, that Blizzard is explicitly approving of this behaviour.
A lot of behaviour is not healthy, but is really hard to prevent without hurting people who are playing properly. Blizzard uses rewards to try and channel people into more appropriate behavior patterns.
For example, consider ganking. There is zero reward for ganking lowbies. No honor, no xp, nothing. Yet people still gank. Now imagine what the game would be like if Blizzard added a title ("the Exterminator") for killing 1000 people who are gray to you. Life on a PvP server would seriously suck.
With rewards, you have to be very careful to only reward the specific behavior you deem good. It's very easy to accidently end up rewarding negative behavior. WoW has had a lot of problems with this in the past. Old PvP system, afking in AV, running Karazhan over and over instead of progressing, etc.
In this case, Blizzard wants to reward efficient levelling. I don't see anything wrong with this goal. But their implementation will reward marathon gaming (aka Catassing) and multiple people playing the same character in shifts, instead of the behavior which is worth rewarding.
Update (reposted from comments because I think it sums up my issue well):
Finally, what the reward is does not matter. The basic problem is the same. Blizzard wishes to reward some facet of gameplay which requires skill. However, the method they choose to measure that skill actually measures a different variable. People obtain the reward by maximizing the measured variable, not the variable that Blizzard wants to measure.
It's that difference which I think is important, and which Blizzard needs to pay more attention to.