Empty rune slotting in Legendary / yellow weapons not possible?

Was suprised to find out it wasnt possible to slot runes in empty runeslots on legendary / unique weapons

examples:
Tarnished wand = born with 3 runes (cant slot a 4th)
The dawn never comes = born with 1 rune only.

They dont show up as options in the menu for rune slotting.

Is this a bug or feature? im just not sure.