Site to Zone Assignment GPO blocking manual user-entries

[OutLawTorn]

I've got some sites set as intranet and trusted via a Site to Zone Assignment policy, and we're finding that this automatically blocks all users adding any other sites to trusted/intranet on their own. Does anyone know of any settings within these policies that allow both setting sites globally and users setting their own?

Cheers

~~Edit~~
Would changing the policy that sets the trusted/intranet sites so that it applies as a machine-based policy then allow users to set policies manually?