There is no way to accurately say you lose/anger more fans/more affluent fans by signing him, than you would gain/please other fans. Rather, it's a value judgment being made, and NFL owners as a group seem to have all come down on one side of the value judgment
You're right, but in the owners' minds it's not worth the risk to find out. Going for the "safe money" (white, male conservatives make up the prime demographic of the NFL's audience) is a much sounder business practice than trying something risky that may or may not pay off for you. That's why all the owners came down on one side of this. Of course there's nothing to say it will stay that way forever. If Kaepernick lays low for a while, lets things cool down, and becomes willing to accept less money, I'm sure there's a team that will then have no problem taking a chance on him.