It’s standard practice for the agency — which is tasked with protecting presidential candidates as well as presidents and other federal officials — to reimburse presidential campaigns for the cost of traveling with the candidates.
In fact, the Secret Service has reimbursed the Clinton campaign, too: $2.6 million so far this cycle.
The difference with Trump is that one of his companies, TAG Air, Inc., owns the plane, so the government is effectively paying him.
The Clinton campaign, by contrast, mostly has been chartering planes from a private company called Executive Fliteways in which the Clintons do not have any ownership interest.
“The taxpayers are actually reimbursing Trump for the travel of the Secret Service agents," said Brett Kappel, a campaign finance lawyer at the law firm Akerman LLP. "It's just another example of how the Trump campaign has taken an unprecedentedly large amount of its money and spent it at Trump-owned facilities."