I think when you start looking at design decisions at this low a level, it doesn't need to make sense at anything more than a balance level. If it works in the context of a champion's playstyle, it doesn't need to fit perfectly into game logic. Shooting a gun or throwing a spear costs mana because it would be ridiculously overpowered for them not to and that would ultimately mean the abilities would have to be much weaker and the champions wouldn't be as much fun to play. Sure, it doesn't make sense for them to cost mana, but by the same token it doesn't make sense for launching a fireball from your hands to cost mana because it doesn't make sense for anything to cost mana. Mana is just a weird number made up for gaming purposes that doesn't represent anything particularly tangible.