The sub-plot with the bicycle killer was out of place (spoilers)

It was really weird. In season 3, he popped up randomly to kill some people, and they ended the season without addressing the plot line. I don't recall Bosch, the main character, even mentioning the killer.
And then in season 4, the killer pops up randomly again, and the next time we see him he's dead, killed by being hit by a car. We don't even see it happen.
And Bosch still has nothing to do with it, with the two other cops tracing the gun found near the body to the bicycle killings. Then they celebrate like they actually tracked down the killer.
I've read all the Bosch books, and this subplot is nowhere to be found there. Why was it added for TV? With only 10 episodes per season, you would think every minute of airtime would be used to squeeze in relevant scenes. Watching this subplot play out, I assumed it would come around to affect Bosch or one of his cases in some way. But nothing.
Like I said, very odd.