Rubie Q noted this in the original Mike Burns to Nashville FanShot. Tom Haudricourt updated his original post about Burns being outrighted to include this nugget from Gord Ash:

Ash also said there is a "window’ after the season in which injured players can be outrighted to the minors. Otherwise, you can’t remove injured players from your roster.

If anyone cares about the nitty-gritty details of what Ash is talking about, I believe it is covered in Section C(2)(a) of Article XIX of the CBA:

(2) Notwithstanding Section C(1) above, a Player who is injured and not able to play may be assigned to a Minor League club:

(a) During the period immediately following the close of the championship season and before the filing of Major League Reserve Lists under Major League Rule 2(a), if:

(i) the Player’s Major League Uniform Player’s Contract does not cover the next succeeding season; and

(ii) the Player, if he otherwise would have been eligible for salary arbitration as a "Super Two" Player (see Article VI(F)(1)), may elect free agency under the procedures contained in Article XX(D) in lieu of accepting the assignment; provided, however, that a Player who accepts the assignment shall not have a right, by virtue of such acceptance, to elect free agency following the next succeeding championship season.

Major league reserve lists must be filed by November 20, so they really could have kept Burns on the 40-man roster for a few more weeks. Burns' contract only covered 2009 and he is not eligible for Super Two arbitration. The passage mentions Section C(1), which forbids assigning players on the major league disabled list to the minors except for rehab purposes.

It is possible to outright injured players after the window Ash alluded to closes, but they must meet the four conditions presented in Section C(2)(b):

(b) During the period immediately following the filing of Major League Reserve Lists and before the 15th day prior to the start of the next championship season, if:

(i) the Player has less than three years of Major League service;
(ii) the contemplated assignment would not be the Player’s second (or subsequent) career outright assignment since March 19, 1990;
(iii) the Player had no Major League service the prior championship season; and
(iv) the Player was not selected by the assignor Major League Club in the immediately preceding Rule 5 Draft.

Burns would not have met conditions (ii) or (iii). The real moral of Section 2(b) is "life sucks as a AAAA player."