I'm still on Jelly Bean so can't say for sure, but from what I've read it seems with Android 4.4 you can set the default messaging app. Only the default messaging app will receive and send messages. So if you use for example, ChompSMS then I guess Google will see nothing.
Just found a post about it on Droid Life.
WRT Apple and Microsoft, I would suspect you have hit the nail on the head. However it is funny as they are doing the same too.
Indeed all mail providers that have a semi decent spam service scan email. They need to do this for us. Otherwise we'd give up using email.