I was actually thinking about that incident. It reminds me of how hard true democratic process is to people who aren't used to it (a la David Graber's Democracy Project). After all, when do people actually get the chance to voice such things on stage at 'mainstream' political rallies? Never, that's when! So no wonder the Black Lives Matter people were a bit rusty at it, and failed to follow a true democratic form in attempting to make themselves heard. However, I think that one of the most important things to come out of that was just the fact that those folks were ABLE to do what they did. You ask why they don't take it to a Clinton or GOP rally? Well, what do you think would happen if they tried taking the stage at such a place? Police would be on them in a second to escort them off the premises (if they were lucky... more likely they'd end up in a jail cell). I know that Mr. Sanders was disappointed that he didn't get to talk, but he didn't use or threaten to use violence against those seeking to make themselves heard with an important message. And that's the very quality that makes him so neat! Give the BLM people time to get their democratic communication/civil disobedience skill honed, and in the mean time appreciate the way Bernie handled things. Tis a good sign.