Agree.No, this is not what the prone condition says, it's not a question of melee or ranged or spell or anything of the kind, it's purely a question of distance of the attacker to the target: "An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage."
As the attacker is clearly the caster, it's HIS location compared to the target that matters, nothing more.
As a matter of fact if you are a bowman and your target drops prone your best course of action is to close to 5 feet and shoot him with your longbow from 5 feet away. Then the disadvantage for using a bow in melee is countered by being within 5 feet.
FWIW my Rogues regularly go prone against enemies with missile weapons or ranged spells.