Technically, your operating system's security model allows this. There's little Firefox could do to prevent it.

This is a historical accident; only toy and mobile OSes have any sort of application-level access control.

There is work to give the user more control over this that unfortunately didn't make it into Firefox 4 [1], but ultimately it's a battle that can't be won; on the desktop, apps can modify other apps. I suspect that this will happen to Chrome too.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=596343

Even though you can't uninstall it, you can disable the extension on a per-user basis. Firefox looks in both per-user and system-wide locations to find installed addons. Only per-user addons can be removed by the user, in the addons dialog. System-wide extensions can be disabled per-user, but not uninstalled (disabling is almost the same thing). To uninstall a system-wide extension, hopfully the publisher has an unistall script, but worst-case, you can just delete the extension folder from the system-wide location. e.g. C:\Users\All Users\Mozilla\something on Windows.

It's an addon

It's not a question of Firefox allowing this -- if you run any program on your computer it'll have access to your data (and hence to your Firefox profile), and one of the principles behind Firefox is extensibility.