I was recently listening to a podcast and they were talking about the three greatest factors to win a game and "possession of the ball" was essentially the biggest one. (Ties in with the 78%).
On a punt return, you can do two things wrong.
1. Have a defensive penalty that provides a team a first down.
2. Fumble - turnover the ball.
Both would fall into a negative big play category.
The positives that can happen (other than just gaining possession) is that you may accumulate additional yardage. In doing so however, you probably increase your chances of fumbling. (Guy in motion being hit by a gunner).
I'm guessing this has been analyzed, and if you are a head coach like PJ who is all about time possession and limiting mistakes, it's an easy decision to just gain possession of the ball without penalty and call it a win. Then let your veteran offense who doesn't turn over the ball much to go ahead and control more clock and break down the defense.
Summary, it's all about avoiding risk / mistakes.