Sadly, that is due to a design choice I had to make when I first started developing the game.

For a simple reason: since you can't know which enemy will drop his weapon and which one won't, not being able to steal a weapon with a melee attack could put the player in a situation he had absolutely no chance to anticipate and avoid. Here's an example: multiple enemies are trying to shoot me, I'm out of ammo, but I managed to dodge bullets and I punched one of the enemies, but it turns out this guy doesn't drop his weapon after I punch him, and I have no way to know before I actually punch him, so basically, I'm screwed, and frustrated.

I try to avoid letting these kind of situations happen when I design games. I like giving the player at least a little chance to avoid unexpected bad situations, even if that's their first try.

Now that's a shame because you had a really good idea, and I was curious to see how your level would have turned out if you could have made what you wanted to. BUT, your level still is interesting the way it is, you have to pick an enemy first and steal his weapon, I picked the one looking at the wall on top of the player, then you have to make a decision : will you kill the shotgun guy? Or keep him for later to steal his weapon ? And obviously you have to keep dodging bullets during the whole process. That's a nice level :)