US-centric here: I feel that uploading a small percentage of a file as a condition of downloading the whole thing may very well fall under fair use - most BT traffic is noncommercial, the portion of the covered work uploaded by "leeches" is very small and probably would be covered by the "30-second" rule often quoted in fair use discussions. The only really arguable point is the "effect on the work's value", but then again an average leech is not uploading enough of the work to have that much of a material effect on the work's value.
