Range deserve a DPS tax because:

1) Unlike melee, they can have sustained 100% uptime while still handling mechanics (melee have to move out of range (e.g. Temporary Current, or other point-blank AoEs, costing them occasional partial losses of GCDs and Auto-attacks). Melee also have to content with positionals and losing partial amounts of potency when mechanics force them to certain sides of the boss. True North helps, but for certain fights it simply isn't available enough to prevent missing positionals. Ranged on the other hand can hit from any angle.

2) Unlike casters, ranged do not have to cast spells and can DPS on the fly while running, dodging, etc. Casters sometimes have to interrupt a cast to move out of the way, and since casters don't have auto-attacks, it's lost DPS.

Therefore since Ranged do not have any real moments where they are disengaged, they should have overall the lowest damage potential of the 3 DPS subtypes.