Why is webcam capture disabled in PiTiVi?

Basically, the feature is disabled because it is broken/unmaintained; it is ancient and isn't properly tested.

The main developers are too busy fixing other high-priority bugs or working on other things, so this requires someone to actually come up and fix it. Patches are welcome! See https://bugzilla.gnome.org/show_bug.cgi?id=624618

In the meantime, use Cheese, VLC, or other tools to record from your webcam.