It is probably to prevent bribery. The SS protects the president but works for the government.
Yeah, the “indefatigable” and “heroic” reporters in the Atlantic article need some basic schooling on how the Federal government does business in the 21st century. Of course, Eric Trump’s number was off but the intent to illustrate the government getting a discount by staying on Trump properties was still valid.
The Federal government has regulations prohibiting it from receiving anything of value for free to avoid even the appearance of bribery. In short, the Secret Service must pay if it receives anything of tangible value from someone. Other posts have noted what it costs to “stay” at the Trump’s exclusive Mar-a-Lago golf club in Florida: $200k membership fee + $15k annual dues + $2k/night room fee. So the $400 - $650/night room fee being charged looks like the government rate.
Here’s another factor not mentioned in the article, this isn’t a conference that the Secret Service just happens to be attending. This is securing the physical location that the President will be staying at. This requires advance teams to survey the vulnerablities of the site, isolate it, provide coverage during the President’s visit. It may require installation of special surveillance, communications, and even electronic countermeasures equipment. And, of course, afterwards, what goes in, must come out. I am sure some locations require entire floors, perhaps even multiple floors, be secured to provide 360 degree protection. Visit the same site frequently enough and it becomes more economical to just retain specific rooms for use by the the Secret Service so that they can install and leave in place their surveillance and communications equipment.
The article, of course, says nothing about how much additional space is restricted from occupancy by the government during a presidential visit. The article makes no attempt to calculate the loss in revenue to the resort due to such restrictions.
One post noted that it was good that the article was coming out now rather than October. The article will be back, that can be counted on. If the White House and Trump Campaign is smart, they will make an effort now to do a full analysis of this question and file it away to be issued when the topic is brought up again.