I think something has been overlooked here.....the fact that a lot of people will simply hit that "Download" button and never, ever go to the details page, which is the only place a comment can be left.
As for requiring a comment, or thanks, or else you won't upload free stuff....well, that's up to you.  I have freebies posted under my old username, some have been downloaded over a thousand times, and for me that is all the "Thanks" I need. :)