I don't even have an iTunes account so I am working on theory here.
iTunes Store: Associating a device or computer to your Apple ID - http://support.apple.com/kb/HT4627
When you turn on iTunes Match or Automatic Downloads, or when you download past purchases on an iOS device or computer, that device or computer becomes associated with your Apple ID. This article describes how to manage these associations."
Are there download options that do not fit under the above criteria such as downloading manually? Would transferring the purchase from the device to the Mac avoid this?
iTunes Store: Transferring purchases from your iOS device or iPod to a computer - http://support.apple.com/kb/HT1848 - only purchases from iTunes Store
Of course this is annoying but it stops people from going around to all thir friends' homes and downloading all their purchases on their buddies' computers.
This is a way around it. I have a big itunes library, it is mine and a close friends combined libraries. what we have done is in my case for my itunes is I signed in and got my library up and running. I copied his song, movies, and tv shows to a harddrive and uploaded it to my itunes. I then authorized my mac to play his media under store-> authorize this computer. Then when need be, I can sign out my username in the store and sign in under his name and download anything new he has downloaded. Then, sign back in under my account. Since it is you and your wife you can do the same thing on the computer just allow sharing or under her itunes use the same library but with her account signed in and your account authorized. I hope that helps.