Do textbook publishers really offer ethically questionable inducements to professors to adopt or retain their book?
There is some hard evidence that textbook publishers do occasionally offer inducements to professors for adopting their texts. The abstract of this paper in the Journal of Marketing Education reports that 


  With respect to publisher incentives, 32 respondents reported that they had been offered and 18 reportedly received an inducement from a publisher related to textbook adoption...


Ricki Lewis, author of Freebies and Other Inducements found that "monetary enticement to adopt a book, although not rampant, does happen, and not just in economics. In science, these extras are encountered in large introductory biology and chemistry courses--but less so in physics and geology classes, which tend to attract fewer students."