But if he is doing what he can to recover the ball, that means the ball has travelled 15 and therefore he can hit. We are talking about before the ball goes 15.
The purpose of the rule is to prevent the team A (moving at full speed) from blowing up a team B (just started moving or moving sideways or standing still) because of a slow roller that hasn't made it there yet. While the guy comes in behind the wrecking crew and falls on the ball as it crosses the line.
If team B is somehow blowing up your team A guys instead of going for the ball, guess what, congrats you probably just successfully executed an onside kick.
I'm not a coach, but if I were, and anyone on my receiving team were too busy just trying to take out someone instead of going for the ball, he wouldn't be in there for a second one.