Riker decided Data was subhuman scum.
Remember, Carey wrote this book so early in the process that all she had to go on were the writers' bible and the "Farpoint" script. And in the original writers' bible, "Bill" Riker's prejudice toward Data was presented as a significant facet of his character, something that would be an ongoing source of conflict between the two of them. Carey couldn't know at the time that it would never be mentioned again after "Farpoint.".
I ran into that problem with my early DS9 novel. A few readers complained that Kira was too harsh to Bashir in that book, but, again, all I had to go on was the script for the pilot episode in which Kira does
give Bashir a hard time. I picked up on that business and ran with it, not knowing that Kira and Bashir would get along much better in future eps . . . .