I am amazed they even have considered it.
They probably didn't. But, all the district attorney has done is refer the case to the grand jury without recommending any charges.
The store owner's case still has to go before the grand jury, who will likely follow the district attorney's lead and "no-bill" the store owner (i.e. refuse to indict him for any crime).
When someone is killed as the direct result of another's action, that's how it works in Texas -- even for the most obvious case of self-defense.