Not so arbitrary.
- Average kickoff return in NFL: 22 yards
- Average kickoff started at 6.5 yard line last season
link
Now move it back 5 yards this season. Means little less than 50% of kicks will be in endzone. The returner will on average get to the 20 yard line. If the touchback was the 15 yard line, I'd say go for it EVERY time - you'd get more yards. If it was the 25 or 30 yard line, I'd say kneel it EVERY time.
Since it's at the 20, it's just right -> gives the returner a decision to make.