First off, I don't think that this judge's decision will stand ... the argument was not that people were not able to defend themselves- which I think would have had merit. But the argument was that they could have foreseen the shooting.
I thought of the same thing .. but alas, no, I don't think it will have any real effect as you (and I) desire.
Now if another person makes a claim that their policy did not allow them to defend themselves (and wins) then businesses would have to weigh their anti-leanings against needing to hire security and the associated costs involved with that.
I could be wrong .... businesses might put in metal detectors or other intrusive measures as well as a condition for entry.