Why are application settings read-only in app.config?

I have some settings in my app.config which I intend to be 'global' - ie. any user can change them, and all users get the same setting.

But unless I change them to be user settings, they are read only.

Why is this?

And how should I go about persisting my app's global settings?

Edit:

This is actually a windows service application which runs as a service as LocalSystem. It can also be run manually by a local admin with argument "/config", which launches a windows form to edit configuration values.

So it will have write access to %PROGRAMFILES% in both situations.

The way I am accessing my settings is thusly:

Settings.Default.MySetting = MyNewValue;

And when MySetting is set to Application (in my project properties, Settings.settings), I get a compile-time error "MySetting is read only".

I am new to this stuff, and have not yet found a very good explanation of how it is supposed to be done. For example, why do I need to say 'Default', and what does that actually mean? I have no idea. If anyone can point me to an app.config usage tutorial, that would be really helpful.