The forum purists live in their own vacuum. Of course, based on the regulars who promote nuclear power, protect the reputation of Monsanto, joined the attack chorus on challenging the motives and bona fides of persons like Edward Snowden... the question of tainted loyalties is very real.
One thing that the purists never mention in their scathing rebukes of Sanders is what the climate is like in this country. In every way but direct mention, a veritable McCarthy campaign is underway.
The long lists of truth-tellers and whistleblowers either in exile, facing prosecution, under house or real arrest demonstrate how little tolerance there is for anyone who DARES to challenge the make-war state.
F.A.I.R has documented how few voices are allowed on-air to criticize the so-called military solution for things like wars & terrorism... which obviously spread wars and terrorism.
When people like Trump, Cruz, Michael Savage, Glenn Beck, Rush Limbaugh are given phenomenal air time to push fascist Hate Speech (under the banner of patriotic fervor), it is dangerous for anyone to directly oppose this military mindset that is so intent on making the U.S. (military) right, and everyone else wrong.
Dr. King was killed because he tied militarism to the nation's domestic policies and priorities... those that consigned millions to poverty.
IF we lived in an era of honest elections, and media was open to diverse voices, and Free Speech still meant something, and journalists were not all pressured into aligning with patently false narratives, and truth tellers were honored, and law and justice meant something... then perhaps there would be fertile soil to "grow" the next crop of Eugene Debs.
Indoctrination, and much of it has come through the now ubiquitous TV set, has gone on for 7 decades. Fewer people read. Mass media uses subliminal tools that manipulate mass emotions. Opinions are expertly shaped... and any truly compelling challenge to THOSE expensive (repeated often) narratives will not BE tolerated.
Just as there are Left Gatekeepers who, for instance, pretend to critique the M.I.C. by analyzing why wars are not being won--instead of challenging the whole raison d'etre of the Beast--the political stage has its gatekeepers, too.
Sanders is stretching the boundaries. THAT is a good thing. For the forum purists, he doesn't go far enough. However, for the reasons I've outlined, it's a miracle that he's gone as far as he has.