Even if he's gone back to labeling himself an independent, hasn't he been dutifully endorsing Hillary ever since he lost the primary?
He has, but that doesn't change that it's functionally backstabbing the organization he just spent several months taking significant advantage of. Well, backstabbing more. Dude got a metric fuckton more out of the democrat party than he put in, and then promptly dipped when metric fuckton didn't end up being goddamn everything. "Hey, thanks for giving me what was probably the largest political boost of my goddamn career! Peace out chumps. What? Help you folks some in return? Nah." Except it was less "thanks" and more "fuck you". Since, y'know. He may be endorsing clinton but he also seems fairly intent on pissing on the group he just used (fucked over, to a fair extent, really) to drastically improve his visibility and crap.
Sanders seems to be intent on making it
really damn clear he ran as a democrat in bad faith, with zero intention of actually doing much of anything for the party except take advantage of the name for a campaign cycle. Doesn't sit right with me at all, y'know? If he'd stayed independent from the beginning it wouldn't bother me much, if at all, (and he probably wouldn't have made the news basically at all, but still), but that's not how he decided to play things out.
I'll be a little less pissed if it actually turns out he does what that aide said, or makes at least some sort of notable contributions to the organization he just got a serious boost from in the interim, but, again. From how this election cycle's played out my expectations of sanders vis a vis that sort of integrity are pretty damn low.