I don't know the answer to that question, legally, because I don't believe any person can be compelled, legally, to release their tax returns. In addition, I find it hard to believe that President Trump, personally, would have the legal ability to claim a "business" tax deduction/write off when he's not technically operating a business, as President. I assume, perhaps wrongly, that a President's tax returns, once in office, are pretty straight forward and uncomplicated by a lot of deductions etc. Now, the trust that is established to maintain his assets while he's in office may try to do so and someone better aware of tax and trust laws would have to answer the question for you about whether or not a "gift in kind", to the government, can be written off as a business expense/deduction.
I'm just guessing, but seems pretty plain to me that President Trump is very proud of the homes/establishments he owns and hosting foreign leaders and dignitaries at these locations is a bit of an ego boost for the man - the tax implications of incremental cost/value of the free facility is immaterial to his intent.