My first thought. John Kerry never even pretended to speak for the United States Government. If Trump called Netanyahu offering more favorable treatment, or worse, or the same, when "he's in office" it would be a crime. It would be a crime for Trump to make representations about the policy of a future administration, unless he were elected, since it would give Israel an incentive to attempt to influence the election. He could be selling his office for foreign support for his election.
See my previous post- Kennedy did exactly that- heck, even Obama did that with the russians
You may recall that Kerry met with Iranian government/diplomats in Paris when Trump was in office. Kerry was trying to tell the Iranians to ignore Trump and stick with the so-called Iran Nuclear Deal (which was and still is releasing frozen funds to Iran ostensibly to “help” them but really was to aid their nuclear program). Fortunately Israel plastered the outside of the mtn the nuke centrifuges are inside, and no doubt will eventually blow the place up from the inside. God willing, some brave operative/infiltrator.
What Kerry did was videoed and he was followed from the restaurant. He was acting as shadow government— and was quite illegal & nothing was done.