I think, this is partially, back side of open source, as semi-commercial activity. What I mean, nothing in this world free, and when You making something, You

I think, this is partially, back side of open source, as semi-commercial activity. What I mean, nothing in this world free, and when You making something, You anyway need some close loop, to make it good, and to retain people (consumers and developers) around Your project. Next in this logic happen strong tie of tutorial to Your library, so tutorial in reality work as ad for You. Must admit, it is possible to be above this primitive scheme, but for this must be magnitudes better than competitors in Your niche.