I suspect it may be due to the nature of True Names, which seem to have special rules or privileges surrounding them - they're the only kind of AC that work on demons, for example. As the precedent is set there, it could be plausibly argued that they're also the only kind of AC that works through the Realms....which suggests that they aren't actually AC's, but rather something else that happens to act as an AC in certain circumstances.
EDIT II - which, in turn, suggests that if you could figure out the True Name for a human being, you could teleport them into the Magic Realm. But there may be a metaphysical issue with free will in that sense - can a True Name truly encompass a creature that can choose to change itself in such a fashion? (Dresdenverse answer: "Yes, but you have to get their True Name directly from them, and it decays rapidly as they change.")
EDIT - that being said, this brings us back to the whole "summoned spirit in a ward" issue. I do agree that airy spirits, when summoned, unless they have some sort of explicit teleport power, end up having to (essentially) walk back to where they were summoned from - in the same way that a Fairy or Elemental would have to walk. In this sense, the Rego Vim summon effect really is just acting like a Teleport power, in the mortal realm.
The disagreement here, then, seems to be regarding the exact nature of a Summon the Spirit of (Form). I'm assuming that it's a native of the Magic realm, whereas you're assuming that it's just another Airy spirit, and thus native to Earth. I think my main argument is the type of sprit itself - Forms are something of an idealized state, and as such their avatars don't seem likely to be hanging around in the mortal world. (In fact, I think that's my main argument - that these spirits seem to be low-level Aspects of their TeFo, similar to Aspects of Daimons.) However, I do agree that this is not explicitly stated in the description, and the actual text simply calls them a "spirit" that "departs" when the casting is done - no mention to where they go, or how they get there.
Regarding your description of how to command a spirit to leave: that...doesn't sound right. Wards explicitly allow the summoner to ignore a ward when summoning a spirit INTO a ward - it seems reasonable that they can be ignored when dismissing them, as well. Which brings us back to the question - how do you dismiss a spirit? And again, I do agree that Airy spirits don't intrinsically have the "teleport back to where I was summoned from" ability. But other spirits do seem to have it.