You could use the same reasoning to ban water-guns, though - and all of them, for that matter. If somebody draws what looks like a gun while being confronted by the police, the policeman won't be able to take a closer look to determine if it's real or not - they will have to *assume* it is, even if it would otherwise, under other circumstances, be obvious that the gun in question is a toy [1]. So you'd really have to ban *everything* that looks like a gun - including, for example, bright orange plastic toys and other such things.

1. This is sometimes done on purpose - a so-called suicide by cop.