This may be considered pushing it, but you could say that the shrieking is loud enough to disrupt verbal communications and possibly flub spells with verbal components (Much in the same way as being deaf can cause you to flub a verbal spell).
The CR 1 is probably there, as stated, if one uses a shrieker as a trap rather then a creature encounter. Also, that low of a whole CR lets them factor in other monsters more appropriate to the level who are "listening for the shrieker to pop" as it were. If they had a fractional CR, then DM's would be encouraged to set up encounters with thousands of them just to smile and flaunt how terrible a DM they are.
