Why do people think it's okay to grief or troll in ARAM because it's ARAM?

I continually see players grief/troll/afk in ARAM arguing that it's okay to conduct so poorly on that basis that it's ARAM. They seem to think that because they're playing ARAM it's fine to grief due to the fact that people don't take ARAM seriously.

How do people rationalize this in their mind? I honestly can't comprehend why someone would sell all their items on pantheon and buy a full ap loadout instead, on that basis that they're just trying to have fun and experiment.

Also, why do people use ARAM as an excuse to not punish players that are behaving poorly. If my teammates are obviously trolling or afk and I ask for a report from the enemy team, I often get a response something like; "Why would I report, it's just ARAm. Nobody takes aram seriously."

I wish I had screenshotted how many times people have argued that "ARAM is not serious." It just pisses me off when I try my hardest to win an ARAM game because I love the gamemode and people take the gamemode as a joke.