Undoubtedly this decision to pull back has been influenced by the protests. With the ads they do have will there be more recrimination from the Left? Will candidates backed by AIPAC face protests at campaign events:
Maybe they are afraid of being attacked by the anti-Israel mob? Better to stay neutral and let Netanyahu take care of business.
There is more than one way to attempt to exert influence. If they aren't running ads you ca nbe sure they are using other strategies to try to get what they want. Its how lobbying works.