I guess that makes sense... but also, the entire gaming community has been conditioned to kill zombies on sight; killing zombies en masse is inherently fun, which is now the incentive. But I understand the logic behind it.
The problem with the current setup is that it's a long ass event (5 minutes? 10? haven't timed exactly), but it's geared towards those who get infected first. There's basically a rush to get infected and infect others in the first minute or so; if you're infected late, you either don't participate or get scraps from infecting MAYBE one other person.
Patient zero has the entire server to infect; meanwhile, patient five has half as many options, AND competes against like 30 other infected. It's likely the person who infects the most is also one who is infected earliest. Which DOES tie in to the goals of the event, but my point is the event is rewarding only in the beginning.
On the other hand, you can turn it into a complete event by making it a regular zombie event. Zombie vs player war, you get rewarded for infecting people as zombies and you get rewarded for killing zombies as humans. There's now a point to being in both sides of the event, instead of just being zombies. Zombies get rewards by infecting early, humans get rewards by surviving as late as possible. Of course, zombies can respawn and be killed infinitely vs a finite number of humans to infect, so rewards should be balanced accordingly, but you get the point.
Bonus points because now there's a reason for people staying in their base to go out and participate.