That doesn't make the purchase legit.
It doesn't make it illegitmate either.
The terms of the Mac App Store allow you to purchase an install apps on all of your Macs, provided you use the same Apple ID. There's no provision that says that you have to stay logged in to the Mac App Store to continue using that app.
Let's say that I own 4 Macs as the head of a household. If my children want to purchase their own software using their own accounts, they can login to the MAS using their own Apple ID. If we want to share common apps (let's say Angry Birds), then I can login to their Mac under my ID to download/install, then logout.
Of course automatic updates would no longer be automatic.
There's nothing there that breaks the terms of conditions of the MAS.
What would then stop me from "sharing" the download with everyone I know ? I do stuff legitimately and me and my GF, while we share a home together, do not share our financial accounts or even any of our accounts.
I suppose there's nothing stopping you from sharing it in this case. But that's no different than the situation with install DVDs.
Your situation with your GF is different. You own your Mac. Presumably, she owns her Mac. If you were to use the same Apple ID to purchase apps on the MAS for both you and your GF, you'd be breaking the terms since they specifically say that you can install on multiple Macs that you own (although they also say that you can install on multiple Macs that you "use").
Breaking the EULA kind of goes against the premise of paying for a legitimate copy in the first place. There was no legitimacy question with the Family pack. It was 5 computers in the household, no accounts, logging in, authorizing, etc.. required.
Cutting down options cuts down adoption. That's a fact.
The terms of the MAS are different than the terms on previous Box Sets where you and your GF would have been covered. It sucks for you, but this is how Apple is currently handling it. I think it benefits more people than it hurts.