Because if you allow cmabt maneuvers to apply to an ally it goes against the rules which said that you can do this maneuver against a foe... ;)
If an ally is charmed he is a foe not an ally so you can use this maneuver against him... (the word against is the key, these maneuver are not meant to be helping you but to hinder you, if you want to help you have to use a maneuver that is not descripted in the book...)
I'll respond to the second part first. Not all charms work that way. In fact, "charm person" says nothing about turning someone against their former friends. Nothing. And then there is the whole illusion argument, or incapacitated argument, or the enemy pretending to be a friend argument... Allowing it wholesale to all creatures is simpler.
Now, as to the first part... Yes, I read the mention of "foe" in the section introduction, but its only mentioned once, and that as part of an EXAMPLE of what is possible. I don't believe it is RAI. If you keep reading, it says "each character or creature...". There is no "friend forcefield" preventing me from dragging an incapacitated ally to safety, or pulling a friend back to the curb from the aforementioned bus. It not only doesn't make logical sense, its also not cinematic. I want the big bad evil guy to be able to shove minions out of his way, and I want the PCs to be able to pull villagers out of harms way. It makes rules sense, and makes better stories.