My point is that there was a very bad stink over this when it started, as it was seen as the president was personally enriching himself. There were plenty of articles about it at the time if you want to look them up.
EDIT: not only does it potentially violate the emoluments clause (and let's face it, violating stuff is Trump's M.O.), but there was also talk that the Trump organisation was deliberately hiking the prices to ridiculous levels so it was costing the secret service far more than it otherwise should. And I think we can both agree that Trump Tower is no Waldorf Astoria.
Oh, and remember he's had over 100 golfing days at Mar o Lago. Even if you're VERY conservative over what that costs - say, $500k a time - that's stupid money that's going directly to the Trump organisation.
Well, i've not missed your point, as someone suggested.
The President can stay wherever he likes. Don't think for one minute that previous Presidents didn't enrich their campaign donors by staying at their properties. So it may not have been part of their own empire but part of someone else's who just happen to enrich their coffers another way. Dirty laundry abounds.
I doubt the Waldorf gave Biden or the secret service a discounted rate for staying there just because he was a Democrat. I do know, per staff, that other guests had to be accommodated elsewhere to clear floors and make room for his stay. I'm going to suggest the government was billed full rate for all rooms. Although I don't know for a fact but based on staff, the Waldorf wasn't doing things on the cheap.
Personally, I don't care where any of the Presidents stay on their travels. There's more important things to worry about.