Um, maybe because they only want you to have one account. Just like if you register a second account here....you get banned!!!!!
Revenue sharing is legal because only 1 pub id will be used at a time. And you won't see ads for your own pub id (as long as you're logged into DP). As far as I know, if you have registered companies then you can have an adsense business account for each one. Each one will have its own tax number and pay its own tax. But you're best contacting google to get 100% straight facts directly from them (and keep this email for future reference).
Really? Make sure to post a thread in the suggestions / feedback forum letting Shawn know. I believe you can only have one adsense account.