A lot of s/w in the Free Software arena is only free for non-commercial use only. So i'd havve been more surprised had they given it away to all.
Despite the occassional looney pundit proclaiming Microsoft to now be an 'Open Source Company' (yeah, right! must have been a good lunch) the only source open is .net and that only happened because they've conceded the OSS development model is superior across many disciplines and they were afraid of losing developer mindshare.
By making it cheap/free to OEMs they guarentee devices shipped with their new baby installed by default. Since only the retail version is free to individuals, the oems will also benefit, keeping them sweet.
By giving it away to individuals they guarentee those user will not go looking for some other free alternative. Same for smal businesses.
Nothin really to gain and a lot to lose by giving it away to Enterprise. Most would struggle to make the change inside the 1 year window in any case.