Apparently I'm a White Nationalist (I haven't ever even gone to look it up, but super Intellectual and perfect Liberal representative AgentFail is absolutely sure I am one, so, yeah...), so I'd say, If I was at any event (regardless if it was White Nationalist event or a BernieBro event) and I organize a large event, and some significant subset of some same believe/some different belief people showed up to lend support to my cause, I'd have to ask myself if getting rid of them is going to essentially sabotage my planned event or not. You have a certain amount you can accomplish, certain time windows, etc. If you are now going to have some kind of huge public argument taking away from your event, is it worth it? Would Event BernieBro turn away Antifa? Shit, who are we kidding, they'd love that they're there and the Media would frame it in the most positive light - if at all.
In this case, the problem is we fought a huge F'ing World War against, in large part, Nazis. What baffles me is, even given 1A, HowTF do we as a country even allow self identified Nazis from even being in our country, much less assembling? The FBI should have been rounding these idiots up, F having the White Nationalists tell them they can't march in solidarity with them...