That's decompiled source and while, for Java at least, it can be pretty usable, it's actually going to differ in many ways from the original source due to optimization and obfuscation. I'm fairly sure neither Mojang nor Microsoft were particularly pleased about this, indeed their use of obfuscation implies they want to make your life more difficult doing it.
I think your point is that the Java version at least is easy to modify, but that's because it's in Java, not because Microsoft wants you to. "Open source" and even "source available" are terms of art which don't apply here. "Very hackable" seems more appropriate.
