Actually I do believe that it's a semi-bug, you actually kick or uppercut the powerup twice before it dissapears. It's true that the manual says you get more for kicking or doing an uppercut, but this is compared to shooting the powerup, and in cases you dont get any ammo, just power up your weapon and very few ammo (10 I think?) in other circumstances. This was intended as actually taking the time to kick, uppercut or buttstomp a powerup is a lot more than just shooting it.

Still, the gameplay differences this causes is pretty harmless, so it shouldn't be considered a key issue.

< theory >
If you were able to buttstomp through a powerup you would also get 50 ammo.
< /theory >