Yet there are instances in the Gospels where they indeed take up stones to kill. I recall one instance, against Jesus Himself, and another against the woman caught in adultery. You're right that it was not officially sanctioned, but I doubt whether the Romans really cared very much if a Jewish person happened to be unoffically offed in such a way.
Cordially,
Even today, I've heard that if you drive on the Sabbath in certain parts of Israel the people will start throwing rocks at your car. But the Israeli state has no formal penalty of stoning.