Apathetic Bystander Watches Man Get Robbed at Knifepoint at Taking Back Sunday Show

A man says he got robbed at knifepoint after a Taking Back Sunday concert last week -- and he's got the whole thing on video ... with some dude just standing by and watching it all.

The alleged victim's name is Brandon -- a sound specialist for bands touring nationwide -- and he shared surveillance footage this week that appeared to capture the crime following a gig he worked in Dallas ... where TBS was playing, along with opening act Citizen.

The clip was taken from surveillance footage -- showing him in a scuffle with another man near the tour buses ... this while another dude is sitting nearby, minding his own business -- even drinking what looks to be a beer, despite the fact there's a ruckus within earshot.

You can see somebody accost Brandon as he is seated on a step to one of the buses -- and it looks like they get into a brief fight, before he's led away ... again he claims this was all at knifepoint and against his will. Brandon tells us his phone was taken during this whole thing.

In the footage, Brandon ends up running back ... and while he appears to approach and talk to the man who sat by and didn't do anything, he insists they never even talked, and he got no explanation for why the dude didn't help him.

In terms of who the other dude was that didn't lend a hand ... Brandon tells us he's not quite sure, but he suspects he was a crew member for one of the bands, because he had a lanyard with what seemed to be a backstage credential or pass.

He tells us he attempted to get the license plate of the perpetrator, but the culprit bolted before he could catch it. He also didn't think it was worth filing a police report over the incident ... so, no, he didn't even call cops about this -- not thinking it was worth his time.

We've reached out to TBS and Citizen for comment about this and to see if the man who didn't do squat was actually with them or not ... but no word back at this point.