Because they stop being human beings made of flesh, blood, and emotions? I think you need to revisit why this started. It started because a rider got his feelings hurt, then lashed out at the world. That lashing out became dangerous! Except, normally when that happens, the pro athlete is told to buck up. You don't find your double standard interesting?
Vudu, you've been reading some good explanations on the forum why this kind of expression by the fans isn't a normal part of this sport. On top of that, why this sport is unique for its dangers. Fans are more respectful in golf. Have you ever heard of booing in golf? No. Because it's normally not part of that sport's culture. Baseball, I will boo the .... out of the opposing batter, why? Because it's accepted, and it's banter not really hate. Have you ever watched a free throw by an opposing player in NBA? The host venue even hands out sound devices to distract the opposing player. I'm not going to get into a philosophical reason why it's acceptable in one sport verses the other, though I think it's been well explained around here, I think the rationalization to the contrary is based on very weak fallacies.
Im not surprised you'd follow Rossi's lead employing a double standard. Simoncelli was considered a dangerous rider, some of his fellow competitors rightfully expressed their disapproval; rightly because they had to race next to him, putting their own lives on the line. Rossi came out in support of Sic, similar to his "this is racing" to Stoner's objections in 08. However, revisit Rossi's take on Iannone after PI05, he said Iannone had been dangerous (btw, VR had a similar take on Elias in 06). I don't recall Simoncelli getting openly booed by fans to express their disapproval. Isolated, perhaps, but not an ongoing activity. So no, this isn't a footballing culture, nor are we to make ourselves feel better by falsely assuming these riders should tune it out, or as you suggest, turn it into motivation.