Warming up during a haka is pretty disrespectful though.
- How so? 
Its a challenge or greeting.
If i said Hello or good luck to you,id find it pretty disrespectful for you to turn around and start doing something else. Not a perfect analogy,but pretty close,so dont go poking imaginary holes in it. Though I know you will anyway.
You can't just make up a scenario to prove your point...