>Why take issue with an unsuspicious app that works as advertised?

  #include <closed_sw_boilerplate_warning.h>

Because you can't prove that the app isn't doing something suspicious. You know what it purports and appears to do, and you know that it needs access to your activity on facebook.com. Even then, it has an auto update mechanism, so even if it isn't doing something untoward now, it easily could in the future without you being the wiser.

Knowing how much of the average user's life is detailed on Facebook, it's one thing that deserves extra scrutiny when it comes to allowing randoms to have access to it.