I was reading the description for Signal Beam and it said that the foe is attacked by a sinister ray. That makes me think that it should be a Dark or Ghost-type move. The majority of Pokémon that learn it aren't even Bug-types...
When signal beam was first introduced, Volbeat was the only Pokemon that learned it by level up. So signal beam was probably intended to suit Volbeat. This can explain its bug type as well as its design in general.
