1) Pickup games usually don't use free throws on fouls.
2) With mixed crowds of folks that maybe don't play all the time, the shooting skill is going to be lower. Thus, people are generally not going to hit shots from beyond the arc as easily. Making those shots twice as valuable as others (as opposed to 50% more valuable) incentivizes players to stay honest on D and not just crowd the key.

At least that's how I see it.