Nerf guns are for people who can't get/afford air soft guns. 
Or, you know, for younger kids who shouldn't use such things. Another possibility is that they want to have a gun fight without getting hurt.
We don't need toy guns in a game. Toy guns don't belong in video games.
So Blockland is a game based off of a toy, but you aren't allowed to use guns based off of a toy in said game, and instead you have to use realistic ones? Nice logic there.