Firstly, there are game balance issues. From a balance perspective, it just makes more sense to have them limited by the augmented max. Especially when you see someone do a Troll possession mage.
Secondly, the augmented max is the limit of what the body can endure without literally breaking apart at the seams. Try to go beyond that, and you'll be doing nasty things like ripping your muscles and breaking bones, and such stuff.
To the first point, yes, I get that. I was more mentioning what I did to justify the Ghost Cartels rules discrepencies, as Shadow Spirits have regeneration and are in full control of the character, not the player, and are well documented as having no care for the body they inhabit, so it makes sense for it to work that way.
If you wanted to put an actual flavour rule on why spirits summoned by a possession shaman can't go beyond the maximum when inhabiting the mage who summoned them, have it be because the mage wants to be able to use their body afterwards, and if the spirit goes beyond the augmented maxes, it's quite likely they won't be able to. Have the reason they don't do it for enemies they possess be that they want to be able to use the critter/person they are possessing, and as such, don't want it getting broken before it's useful.
To the second point, you will see I mentioned that already. I see it essentially as magical overclocking. I figured I'd bring it up as how I would deal with the issue, this being a 'house rules suggestion' thread.