Why do people justify bad behavior as "It's just a bot game"?

I'm playing a match with a guy with an incredibly inappropriate summoner name, and he also goes around last hitting stuff that I was trying to kill (Like the enemy blue buff, after I had it down to like 5% hp he came over and stole it, claiming he "didn't see" me). He sat and watched me get 2v1'd (It was a co-op vs bot game, so there were 2 enemy top laners) and I got them both low (I couldn't secure a kill because they kept moving away from me and I couldn't close the gap) he literally sat in the bush for 10 seconds watching me die, and as soon as I died he jumped out and got a double kill on the two <10% hp targets. If he had helped at all, I wouldn't have even come close to dying.

At the end of the game in which I was harassed the ENTIRE MATCH by this guy, I asked my team to report him, and everyone is like "It's just a bot game", "It's only bots", "don't take it serious, they are only bots". Like, what does that have to do with anything? Abusive behavior is bad regardless of who the opponents are!