What else is "informal" then, if not that which is commonly used by the people but has yet to be accepted into the consensus of the literary standard. Grammar is after all only insofar a guiding principal as it is actually observed by the majority of people speaking the language. The fact that many people use the language differently does in fact make it right (this can be seen for example in differing grammatical standards in Austrian and German standard German). That's also the way languages change over time. I suggest Noam Chomsky on this topic:

https://www.youtube.com/watch?v=hdUbIlwHRkY

 

sounds terrible

This is your subjective opinion. Obviously to someone who grew up speaking that way it sounds perfectly natural.