Thanks for pushing through the initial option-shock.  I think this other forum post will answer your question.  Let me know if it doesn't.

The "Restricted editing elements" stored in the "Content Maintenance" section of Role Scoper Options will be hidden if the user is less than an editor for the object, or if they don't have any blog-wide edit roles (i.e. they are a WP Subscriber).  You can add additional elements to these lists by viewing the html source of your edit form and noting the div IDs.

I will entertain suggestions for how to make this functionality more convenient or versatile.

Since Contributors can post in any category by default, your primary need is to restrict the other categories by defining Exclusive Section Roles.

• Go to Roles > Exclusive Sections

• Select the Post Contributor role for all categories.

• Confirm that update mode is for blog-wide assignment to be “ignored” for selected roles and categories.

• Click update

Now the Post Contributor role is an Exclusive Section Role for all categories, meaning that it is only honored for users who have the role assigned explicitly for the category (or for the post).  The Section Role you assigned for [WP Contributor] should now have the effect you intended.

In the upcoming new release of Role Scoper, defining these restrictions will be more intuitive and convenient, including the ability to define default restrictions for all categories.