If I had to take a guess, I'd say it was simply a matter of perception. No government/government agency wants to be seen as soft on illegals; the general public isn't happy when such people receive any form of benefit, which a work permit definitely is. So is being allowed to remain in Canada!
They can justify, and I believe most people in Canada accept, giving an illegal person PR status after an in-depth application verifying that they are the genuine spouse of a Canadian citizen/PR. No one wants to split up families. Agreed, but clearly some don't want those without status to be able to provide for their families...just like those with status that will get the highly coveted OWP.
However, it would be harder to justify giving a work permit to any person that is illegally in Canada, especially if it's issued based solely on a submitted PR application that hasn't actually been verified.
Perhaps, but if a person without status is merely trying to defraud CIC, their Canadian sponsor would/should be legally liable if they do, since s/he will be signing the agreement to sponsor them.That can easily be seen by John or Jane Canadian (who knows very little about the immigration system) as a "soft on illegals" action. Perhaps they should educate themselves on the fact that those without status were eligible to apply for Inland sponsorship, just like those with status, that may be in a fraudulent relationship. The OWP with AOR really shouldn't be based on anything other than that.
As for such people being able to apply for an OWP after AIP, perhaps CIC allows it because they know by that point that the relationship is genuine and the person isn't trying to use the PR app simply to get some form of status in Canada. Same comment above would/should apply here as well.