The amulets are two halves of an extremely powerful protective charm. If they are together and are activated, the bearer becomes invincible.
But, if they’re apart, they kinda-sorta protect against evil, but not against very powerful evil. Like Balthazar, who just waltzes in and grabs it.
And they were divided between two local covens “for safekeeping.”
So lemme get this straight: something that is supposed to serve as protection, and that is extremely powerful at protecting, has been divided in two, thus reducing its effectiveness, and then given to two civilians to keep, even though the fact that “only a very powerful evil” could bypass the protection means that, logically, the bearer of the half would only ever have to contend with “a very powerful evil.”
“Hey, we have this thing that’s really good at keeping evil away. What shall we do with it? I know! Let’s make it much less effective at keeping evil away, and then give to random people!”