Problem/Bug WorldGuard (Build=deny disables Pistons?)

Hi,
im having some problems with WorldGuard.

I want that players can't build in the region "__global__".

But when I set the flag "build" for this area to "deny" then I can't use pistons, sand and gravel-physics are not working anymore, etc. How can I get the pistons,sand,gravel to work?

PS: I already tried the "pistons"-Flag but it didn't worked.