Then you're part of the problem.
They won't get caught out if the rest of the team pressures the map elsewhere. If they do get caught, then either your team is really behind because 1 person just killed them... or, you haven't pressured the map enough and the enemy team can send 4-5 people after them.
Normally they'll send 1-2, and it needs to be someone who won't die 1v1, or enough to overpower the split pusher so that they can't keep pushing. This gives the rest of the team on the split pusher's side an advantage and an opportunity to push an objective. Udyr for example is amazing at split pushing because of his ability to wave clear, move quickly, tank and destroy towers, and retreat from multiple enemies at once. He's also quick enough to draw out 3 enemies on one side of the map, and then regroup with his team for a teamfight, so it'll be a 5v3, in some situations.
The issue here is that the people yelling at others to group don't understand the game. Surely, not everyone split pushing is right, and sometimes they're just not grouping and actually hidden on the enemy map and not applying pressure anywhere. But when it comes to split pushing, it's an incredibly effective tactic that will give a huge advantage to the team that responds to it properly.
When you're 4v3 or 4v2 when grabbing baron or taking a tower on the other side of the map, you've gotta take advantage of that. The split pusher will almost get caught out every single time if the enemy is free to send everyone to intercept. But if they're pressured in two places, then they've got to either lose an objective and group as 5 to take out your team (and you shouldn't engage 4v5 and then yell at the split pusher, it's not his fault ya'll done messed up), or they can split up and send 2 people to intercept the split pusher and 3 people to contest the objective.
Split pushing gives you and enormous advantage. For example, I can push out two lanes before a teamfight, and before the fight is over, the enemy team now has two turrets being threatened by our minion waves, and they'll fall if they don't do something about them. They might win the teamfight and lose 2 turrets, or they might lose the teamfight and lose 2 turrets. If I were the enemy team, I'd need at least one person to take care of those turrets. A champion like Udyr however can push both top and bot and group for a teamfight and push down mid. Now the enemy team must send 2 people, and your team gets a free inhibitor in mid.
It's complicated, but... it wins games. One game, I was a mid lane mundo, and the enemy pantheon had roughly 30 kills and 10-20 assists. Our team thought it was over. I AFK split pushed bot lane, and then rotated top partway through. They always had to send 2 people to deal with me. I ignored everything my team did as they grabbed a few dragons and pushed the remaining lanes, and eventually I grouped for a baron and we finished the game. We were going to lose that match. The enemy team was steamrolling us, but, Mundo will go where he pleases.
