Because of the security implications of presidential travel, it is not unreasonable for taxpayers to pay for a reasonable number of trips for personal reasons.
Fundraising trips are a different category, and I believe they are treated as such, to a great extent.
As I understand it, fundraising trips are supposed to be paid for with campaign funds, not taxpayer money. Whether this ever happens, I have my doubts. But when the president travels on other than official business, we taxpayers are not supposed to be billed.
On the other hand, if the president goes somewhere on official business, and just happens to squeeze in a political event, we taxpayers are paying for the entire trip.