You are right that according to its Physiology that giving Tail Glow to Ampharos makes sense. However, an Ampharos with Tail Glow would probably be a bit over-powered. It increases Special Attack by 3 stages, so would become an awesome move on a Special Sweeper like Ampharos.
The Pokemon that actually get Tail Glow aren't really that good. Volbeat is average, at best, with terrible base Special Attack anyway. Manaphy is legendary, so isn't too bad, although I have never seen it used competitively. However, Almpharos has a better Special Attack than either of these Pokemon, so after 1 Tail Glow even resisted hits will do serious damage.
Hope this helped.
