Just change it to point at the root /Library/Preferences/ folder:
#!/bin/sh
defaults write /Library/Preferences/com.apple.finder AppleShowAllFiles FALSE
killall Finder
Although the com.apple.finder.plist file doesn't exist normally in the /Library/Preferences/ path, the above command creates a new one with that single setting in it. As long as its running as root, it will be able to create the file.
If you don't specify a proper path, just using "com.apple.finder" (or whatever plist you're trying to write to) will only apply to the account actually running the command. In the case of a Casper Suite policy, that would be root. If you do it as yourself while logged in, it looks to the file located at ~/Library/Preferences/
I'm not 100% certain if the above command pointed at /Library/Preferences/ then applies to all accounts, but in my tests, restarting Finder will show me invisibles if you change the above to "TRUE"
Last thing to keep in mind is that, unless you convert the above into either a System Level Enforced MCX or put it into a profile, an individual user's com.apple.finder.plist can potentially contain an AppleShowAllFiles TRUE setting that would override the above "FALSE" one you've placed in the root /Library/Preferences/ folder. If there is no setting in their plist, it uses the one in the above location, otherwise, it will use whatever is in their local plist.
Does that make sense?
