They implemented WebGL in IE, so sitting this out would just leave them having to implement a standard without input into the process.

Also, for old monopolist Microsoft, standards were a bane because they meant smaller players could interoperate with its software, and even sometimes influence its future via standards committees. For new underdog-in-certain-markets Microsoft, standards are a blessing because it means they can interoperate with larger players' software, and even sometimes influence its future via standards committees. I'm not sure how much that's taken hold culturally within MS, though. (And, of course, there are many places they're still not the underdog.)