I have no Idea why Bernie supporters get so upset about this.
Because it's incompetent, its intellectually dishonest, and has accomplishing nothing positive except satisfying pseudo-progressives wanting to prove how radical they are. Not voting for Sanders because some of his supporters are reasonably frustrated with BLM's show of being ineffective at doing anything but making a lot of noise this election cycle is ridiculous, for starters, but let's break it down.
Clinton's taking kickbacks from the fucking prison industrial complex, how much more anti-black can you get? It doesn't matter if she met with some activists compared to what she's doing now and what her record is. Promises don't mean shit, and I say that as a diehard wool in the cloth Obama supporter way back from when he first announced his candidacy. It's been dissapointing, but its going to take 5-6 Obamas or Democrats like him to really push things back to something reasonable in this country.
Sanders is getting some fucking momentum going finally and no matter how much Sanders says or gives the stage to or his record - it feels more and more like until he's on his knees flogging himself senseless these protestors won't be happy. Read the DailyKOS accord I posted earlier, he's tried to engage, for god's sake. Instead he's met with hyperbole about how their going to "burn this shit down," which A) they won't and B) in elections you have to actually persuade people, not just be the loudest and the mosst correct.
You know why the Right has so much power in this country? Because they manage to get their shit together and unite, and the Left is going to have to suck it up and do that, BLM included. Yes, that is a fucking bummer to hear, but the continued electoral success of the Right across this country is going to affect the oppressed, especially black America, the most. There's plenty of constructive solutions this early in the game to do, plenty of real organizing to be done, plenty of actual disruption that might make some change happen.
Forcing a confrontation with a crowd and then claiming disrespect because they won't get on their knees and grovel for you is disingenuous. If there was an attempt to honestly engage Sanders he's made it more than clear he's willing to listen. But this isn't happening, and it's on the record not once, but twice now, and Hillary is seizing every opportunity to profit from these. She intentionally skipped Netroots Nation and she has ex-Blackwater running her security just like last campaign, keeping BLM activists out of events, yet there isn't a single news story about this?
BLM looks weak because they're preying on who's accessible and Sanders has made himself open to this, because he's trying to run an honest campaign here, and BLM is fucking shutting it down - which would be great if it was targeting actual bastions of white supremacy, not the guy with the record both then and now of speaking up for black causes. Insisting Black Lives Matter is beyond criticism and that you can't blame them creates a chilling effect and is ultimately toxic to the larger political discourse taking place here.
Now you have the Left raising questions about the legitimacy of BLM if not outright writing them off - this is the opposite of what you want to happen, unless all you care about is being the most right when The Trump House begins 8 years of making shit even worse than any of us could possibly imagine.
Black Lives Matter has been the largest news story of the past year and this is only pissing away what momentum and relevancy they had. Honest, meaningful political discourse is a two way street, and Sanders has tried to open this channel up on multiple occasions now, regardless of whatever fiction people are construing about the series of events taking place at these disruptions.