Will the true God please stand up...

For one, God cannot lie which is hard enough to do when talking about the past. He does not lie about the future which equates to keeping every promise. Throughout the entire scriptures, God states the results of both sides of each possible choice in great detail. Then as Israel makes their choice, the stated consequence is played out.

If he were not to punish them, he would be just another known liar. We would need to have him swear on penalty of felony perjury in order to get the truth from him.