Adjuncts do not generally write textbooks and get kickbacks for them. They choose one of the 'standard' textbooks, and use the online problem set for that book. They do this not because they get compensation for it, but because it's the path of least resistance.
I find it kind of sad that you would speculate like this, without any specific examples (let alone proof of a general trend!) about the motives of the adjuncts. :(
This post seems to provide evidence that people who do not traditionally write textbooks (not the top hundred faculty members in a field) are being afforded a new opportunity by textbook companies to "write a book", which either means "pick and choose some portions of the standard textbook" (discussed elsewhere in the comments) or "publish a 'course reader'-style collection of readings as a book" (discussed in this blog post).
It seems to me that this new opportunity also represents a new revenue stream for more faculty members than currently receive textbook kickbacks.
I don't mean to suggest that I have any special knowledge of the motives of adjunct professors in particular — basically I just read the Chronicle of Higher Ed like everyone else. This is a well-trod area: see July 2008's "'Custom' Textbooks Raise Money -- and Questions of Ethics" (http://chronicle.com/article/Custom-Textbooks-Raise/41288), February 2010's "Format War Heats Up Among Publishers of Electronic Textbooks" (http://chronicle.com/article/Format-War-Heats-Up-Among/64323...), or October 2010's "As Textbooks Go Digital, Will Professors Build Their Own Books?" (http://chronicle.com/article/As-Textbooks-Go-Digital-Will/12...), to pick a few examples.