His campaign, not taxpayers, should pay for his travel and events. He almost always injects politics into what should be a non-partisan presidential speech. We can go back to the beginning of his presidency when he did this at the CIA and Boy Scouts appearances. He has not changed and won’t change. I hope he is sued and his campaign has to pay.
The link above takes you a to a good interview with the communications director for the Citizens for Responsibility and Ethics in Washington (CREW).