More like asking Green Lantern for his ring.
Basically the same as trying to steal Green Lanterns ring.

And I'm sure that attempting to incinerate the guards was the only imaginable way to reject this demand.
The Guard tried to TAKE the staff.
(ref http://www.neorice.com/hoh_2335)

Without it Piper is just a small girl. Its basically the same as assulting her.
In a situation where someone already put a price on her head. And where she suspect her Uncle of being scetchy.

So yeah. Basically a quick fire spell was the only way for her to defend herself.
If anything she showed remarkable restraint in letting the moron keep his head - hair.