i'd have to agree. This isn't shady or just for idiots. Its for those who don't have the capability to download such large files from the net. Someone with a 56K modem and noone else to get the file for them will gladly pay just $5 for the disk, be it demo or not, that is if they really want it.
And yes, I bet the $5 is to cover the expense of getting it to the shelves for peoples to buy. Not only that, it may be for market testing purposes.
Rather than laugh at someone for buying it, why not offer to download it for them and mail it to them. Then again, for all that work, you might want $5 too.