Slimjet extension packers

Slimjet will not accept extensions that have been added from a .crx installer packed by Slimjet itself. It seems that the extension has to be installed or packed from What is the difference? Is there some flag that is set somewhere in the profile that tells the browser that the extension was not installed or packed from the webstore? Why is there a built-in facility for packing add-ons if they are to be disabled on restart? Themes will be installed from .crx files without a problem.
An extension called GrabCRX will make a .crx file from the webstore page that will be accepted. What is it doing that the packer in Slimjet is not doing?