Often they write a special report or an ebook them selves (a pdf-file) to give away as a bonus, or outsource it to someone else.

Well, in my opinion, i think, most of the experience will make the bonuses by themself...
Since its guarantee the copyright of the product...

Or maybe they bought other's product, but of course with Rights (PLR, MRR, RR, etc)

Just my opinion though :>