I'll ignore the offtopic suggestion (hint link to a new thread with suggestion) as this is about static menu only
The reason why it isn't static is probably because it worked like that without any additional changes.
The other thing is that it is the same for any ingame menu, isn't it? Camera moves when they are open and it would be weird if it didn't move for say the F1 menu.
What's the advantage to freeze it for weapons menu only?
The camera would jump suddenly when you choose a weapon depending on where your mouse was when the weapons menu appeared.