Because then you are relying on a specification that's not in the spec. A different interpretation means that your code no longer works. Therefore, you've effectively created a new language.
The trick is to try to bring together all of these dialects in a way that everyone can in fact understand. This is why writing language specifications is _hard_.
And even afterwards, you can still choose a dialect over the standard. You'll just lose a certain segment of your audience. That may or may not matter for your purposes.
I'm also not saying that's inherently a bad thing.
I'm not sure this is helpful though. We've accepted "literally" also means "not literally" without creating a new language.
http://waste.typepad.com/waste/2014/07/on-a-popular-misconce...
It's just a figurative use of "literally". It's like saying "He's such a baby" of an immature (<- also figurative!) adult. You aren't saying that such an adult is actually a baby.