Speculation and theory are all fine but they are already nullified by the law. If you really want to understand why these and other laws create exceptions like this, I would start doing research on the subject. And if you wish to change the slow poke laws to coincide with your theories than I would suggest gathering data for review. But as it stands these are the laws they have in place and to save yourself a ticket or worse an accident, I would recommend keeping to the slow lanes.