The only reason Lehmberg didn't resign is because Perry was going to get to appoint her successor. This may seem benign, but the capitol of Texas is Austin, which is in Travis County. This particular DA has jurisdiction over state officials. And Austin always puts Democrats in that office.
Lehmberg always made it clear since her arrest she wouldn't seek reelection as part of her penance, and she did sit out the primary and isn't on this fall's ballot.
This isn't the first time Perry has overstepped his powers; but it is the first time someone was able to nail him on it.