I have magazines from the 1910s-1920s advertizing these song publishing services. I wonder if some of these printings being laughed at contain material useful to folk song collectors.

The printers who will publish your book if you pay have a long history, probably going back to the beginning of mass printing. The printer would promise royalties on copies sold, but of course none was ever sold. The author would end up with a few boxes full of his books and short of a piece of his savings.
Another ploy was, for a fee, to place your brief biography and photo in the back of state or provincial history books that were peddled door-to-door; a compendium of "Notable Citizens of Our Great State." I have histories of this sort from 1890-1920 from my state and the province where I now live. These books have become important to researchers and historians and some command a pretty good price.