When the self-appointed founder of the term 'alt right' conned the term and movement to be about white supremacism and neo-Nazism it's really not that hard to see why 'opponents' to the alt-right consider the alt-right to be Nazis. Then the election happened and the memesters got involved and suddenly when before people were just like "fucking feminazis" now they were parking their carts with people who identify as actual honest to god white supremacists that believe white people are in danger. 

It's less a tactic and more kind of the mantra of the goddamn movement. People aren't using "Yeah but you guys are Nazis so" to dismiss the alt right when an argument isn't going their way they're doing it because [i]the alt-right has self identifying neo-Nazis in it[/i]. And a reason why people are saying "Trump is literally a Nazi/Hitler" is because when the President appears to be soft on them (because they're in his corner) it's a show of support. No, he's not literally a Nazi or even Hitler, but when he's got friends who are like "Yeah the alt right? Some good people there." it raises the eyebrow because are those really the people you want to associate with? 

It'd be just as insane of a President saw Antifa riots and went "Yeah those are my people." They're extremists. Not all conservatives are alt right and not all liberals are antifa. If someone says "I am part of the Alt-Right" then call me an asshole or whatever but I'm not really going to want to associate with them. I might want to have a discussion with them, sure, but at this point it's kind of clear that the Alt-Right aren't the most tolerant when it comes to people who have differing views.