Default [Q] Why don't some apps work when moved to /system/app?

As the thread title asks. For instance, I can move most of Google's apps to /system/app with the exception of Google Earth. Why won't it work where the others will? External libs? I can also put some larger apps there such as Grabatron, and have them still work, but other apps (like Edge or Osmos HD) won't work at all.

Any ideas?