Why can't you deauthorize just one computer?

Question

Why is the only choice in the store to deauthorize ALL computers with an appleid/iTunes store account?  My husband took my old mac book pro and removed all my info on it, but did not deauthorize the iTunes account first.  Now, my phone won't sync with my new computer because we're up to our limit on devices with that iTunes account (try having one account since day 1 of iTunes, and now multiple computers and devices.
Isn't there some way to tell iTunes to stop associating that mac book pro with my phone (which is what it seems to be doing)?
And word to the wise -- do not let anyone use an old computer until you've deauthorized it.  Once they start ripping out your data to put theirs in you really are totally (******) if they do it wrong.

Answer 1

The only thing that makes me nervous about that is this -- we had an iTunes account forever, and when the Cloud went live, they made us change the login and update our account information.  Our original login id still syncs anything associated with it for updates (apps), even though we now use the new id for purchases.  It's rather beautiful that it works like that -- asking for one login to update apps prior to the Cloud, seeming to know that it goes with those ... however, I shudder to think if we tried to reauthorize to that original id if the iTunes store would recognize it as a valid id, which could jeopardize all the original music and app purchases (years and years and years of legally purchased content).
We're going to have to try something else.