It must have been associated with another id at some point in time. This isn't the association for playing content that's been around for years to protect music and videos. When Apple introduced the ability to download past purchases, this started, For autodownload o fpurchases, download of past purchases and now itunes match, each device for those functions can only be associated with one Apple ID and you have to wait 90 days to change it. Our household has 3 or 4 id's and multiple computers and we were hit with this. Even on one machine with multiple OS level login accounts, only one id per machine was allowed for autodownloads. So two kids sharing a machine with two apple id's are stuck. Only one could activate automatic downloads and past purchase downloads. My hunch is this came about as part of the licensing for iCloud and iTunes match to prevent piracy.
Do you have another id that you use? You can sign in to itunes with that and check the account. It has a section that lists the devices associated with an id for auto downloads and itunes match.
More info here : http://support.apple.com/kb/ht4627