Apple picked PNG because Microsoft didn't. And screencap image formats were hardly going to be an industry deal breaker. BMP files are so obvious as to really not give any leverage to Microsoft. Any patent on the silly thing would be overturned("Method for displaying a picture by lining up the colored pixels in order?").
Apple is pro-open-formats only if there's no way to get a lock-in. Apple is a patent holder with MPEG-LA, hence their preference for H.264.
Remember that there's nothing preventing you from installing Theora and Matroska codecs on your Windows box, same as with your Mac. Sure, media player will push you towards wma, but Apple will push you towards Apple Lossless.
"Microsoft has a history of always creating their own formats even when a multitude of alternatives already exist"
- iPad USB dongle
- iPhone/Pod connector
- Their old practice(no longer the case) of locking their RAM so only their RAM would work on their hardware
- The stupid one button mouse(yes, it's no longer the case, but it was).
Not saying MS is good. They're not. But neither of them are. Trying to say "At least Apple..." these days is a tricky game.
Also, MS has stated that IE8+ will handle WebM just fine as it can be installed via Media Player. Opera stated support. Mozilla breathed a sigh of relief and quickly said the same. Apple(or at least Turtleneck Prime) is the only one that seemingly indicated they wouldn't support it by throwing some lawsuit FUD around. They may yet, but the response wasn't exactly positive.