So you reward the O for playing careless?
No. They're treated just like they would be otherwise. There is no reward. They don't get extra yards. Just as it is somewhere else on the field if they're careless.
It's a fumble forward out of bounds. There's no recovery, so neither team gets any advantage for where the ball goes out of bounds. It simply goes back to where is goes if it happens the rest of the entire length of the field. We don't reward the defense then. Only recoveries are rewarded.
By changing the rule at the goal line, you're giving an extra bonus to the defense for being driven on the rest of the field first. Or punishing the offense for driving so far down the field before the mistake.
The only exception should be if the ball goes out the back of the endzone as opposed to the side. In that situation you're dealing with a ball that normally would have been in play and could have been recovered if it were anywhere else on the field.
That's not the case for a ball going out to the side.
If an offensive player dives for and reaches for a first down at the opponents 23 yard line, and the ball gets knocked out and goes out at the 21 yard line, why is it not considered rewarding the offense by letting them keep the ball in that scenario? Why is that not considered punishing the defense? Why do we change the logic because the offense drives another 20 yards first?