Karl didn't give many bad reviews. Reading a lot of his stuff and his friends and counter parts (Johnny Pounds, Art Lassagne, A.T. Evans, Bill Mahan, etc.) it's pretty obvious he had a sort of "you scratch my back, I'll scratch yours arrangement with many other noteworthy folks in treasure hunting, particularly those publishing. What you see is folks swapping articles written for free publishing and both sides frequently mentioning other publications in an environment one might see them as competitors. Karl also reviewed books that were sent to him, meaning he'd get two copies of his own for his personal library...so undermining people that might publish later would have hurt him, particularly since he was acting as a distributor for many of these books.
He did something similar with metal detector reviews...he'd name positive reviews, but would speak cryptically about manufacturers who were turning out bad products or those that had blatantly stolen designs from other manufacturers. He never, as best I can tell, named a brand he had negative feelings about though he'd speak a great deal about why some brands were band (without naming them specifically).
Another way I've seen these arrangements play out is in subscriptions. Some years back I bought a bunch of H. Glenn Carson's magazine collection, all of them addressed to Carson Enterprises. Long story short, he'd write articles or do a regular column in exchange for free copies.
