I do find it funny that people are still blaming the Microsoft IE team for IE6, even though it's probable that most of the team that made it and marketed it are long gone.
MS have stated time and time again that they pay more in licensing fees to MPEG LA than what they make from it. Therefore it'd only be detrimental to other members of the cartel if they go after Microsoft for implementing WebM.
The reason for not implementing WebM are as simple as MPEG LA and Microsoft not wanting the format to succeed.