You can
restrict the editing, but if you don't intend for your group
of readers and your group of writers to overlap heavily, then you
shouldn't be using wiki software. There are better ways for a small
group to collaborate of material intended for a much larger group to
read.
I also agree with this. The "letter" of releasing Mediawiki to the
public mean anyone can use it for any purpose, but the "spirit"
dictates that if you don't intend to have people edit it in a
"Wikipedia-style" fashion (ie We assume good faith here, so you start
out with these editing privileges), then you need to go elsewhere.
It doesn't have anything to do with the release of the software, it's
just a matter of using the right tool for the right job.