(1), the MWC couldn't "give" BYU their third tier rights, because the conference signed them all over to Comcast. The MWC didn't own them anymore. It was Comast that denied the rights (Hair Thompson screwed it up when he negotiated the Contract).
(2) Boise got to keep its third tier rights because it left the conference, and then rejoined. It demanded them as a condition to membership.
Here is the Boise Terms Sheet:
http://www.usufans.com/Forums/download/file.php?id=1724