Jay -
The covers I am familiar with allow the switch to be in any position while the cover is open. When the cover is closed, it forces the switch to move to one position. The whole point of the cover is to give the operator a quick and easy way to determine that important switches are in the desired position.
P.S.
They do sell a toggle switch guard intended to prevent inadvertent activation of a switch and it allows the switch to be in any position. Here's a link to a site that sells them:
Click here.