I think the point is you can't have the people you're sending the emoji to to use your open source rendering of choice. That's how it's corporate controlled. In contrast, corporations don't have as much choice in how plaintext glyphs render, without making them potentially unrecognizable.
Couldn't they just make their font have a ligature so that :-) turns into whatever they want? That's how some programming fonts make stuff like -> or != look different.