These things are rewards that one can spend Tome Points on.
Unfortunately, they have two major design flaws.
(1) They do not work on Companions of level 20 or above, at all. But one might WANT to use them on a max-level companion - for instance, to teach them a new kind of weapon or armour, and level their skills in that, which requires continued gain of XP. (Especially if you have all 8 companions at their character level 20.)
Proposed solution: Instead of disallowing it outright, give an "Are you sure? (Y/N)" prompt, if the Companion is level 20 and you use a Commendation.
(2) They do not stack. 5 Major Experience Commendations occupy 5 inventory slots instead of 1.
Proposed solution: Make them stack.