> the net cost of copying data is effectively nothing.

Since the cost of including the author and the link to original work is so cheap I dont see why not have the courtesy of adding that info. They're are already doing the work for free, recognition is the least you could pay in respect for it.

Granted, sometimes things are a complex tree of authorship transformations and it's an interesting question of how far back you d go for that but sometimes its pretty cut and dry too.