There is a law forbidding the use of taxpayer funded entities for private purposes. Doesn't matter what the office is.
https://www.justice.gov/jmd/misuse-position-and-government-resources
Remember when Trump said he would turn his enterprises over to the kids:
http://www.politico.com/story/2017/03/eric-trump-sharing-business-updates-father-236464
“I am deadly serious about that exercise,” he said. “I do not talk about the government with him, and he does not talk about the business with us. That’s kind of a steadfast pact we made, and it’s something that we honor.”
But nearly two minutes later, Trump admitted that he will keep his father up to speed on some aspects of the business.
“Yeah, on the bottom line, profitability reports and stuff like that, but you know, that’s about it,” he said, adding that the updates will be “probably quarterly.”
“My father and I are very close,” he said. “I talk to him a lot. We’re pretty inseparable.”
Trump and his White House have argued that the commander in chief is exempt from any conflicts of interest but that he handed over management of his company to fully focus on his administration.
Lawyers for the president have said the company won’t enter any new foreign deals and that any new domestic deals must be approved by an ethics adviser. Ethicists, however, have repeatedly criticized the arrangement, which they contend doesn't go far enough to separate the president from his massive business empire."