Thay have explained this several times. AF's will never be dyed because the design is connected to the job lore and feel. A White mage AF can't be black and purple.

Dungeon drops like the battlemage tunic are suposed to be unique items so the color and designs are fixed, crafted items on the other hand are created by people all over Eorzea in mass so those can be dyed or bought of any colour.

Bascially, it's a lore thing. Some may like it, some may not but that's how it is and don't expect it to change.