I think everyone understands all that, and no one would have blinked an eye if Leno had joked around with Bob Dole, or Fallon with Romney.
Trump is an entirely different entity. He caters to hate groups and bigots and genuinely awful human beings.
Would you be ok with Fallon joshing around with David Duke?
Definitely not. But you notice the media, the Big Boys, haven't called out Trump on direct racism.
Even the NYT, very critical of Trump, hasn't outright said it. They've had OP-eds and columnists say it, but not the paper directly itself. Same with the Washington Post, WSJ (LOL), and others.
Trump has been accepted the by mainstream and Jimmy is as mainstream as they come. Unlike a David Duke, who has been rightly shunned.
The critiques of Trump's bigotry have been smaller outlets and individual people. Not ABC, NBC, CNN, etc.