Why are people absolving the black bloc of basic personal responsibility?

This post builds on ideas from my G20 braindump and occurred to me while reading this post from James Bow, but can be read without prerequisites.

A lot of people have been asking lately "Why didn't the legitimate demonstrators do anything to stop the black bloc?" (And, indeed, that has been used as justification for Queen & Spadina.) Some have asked in return why the police didn't do anything to stop the black bloc. Some have pointed out that that people did try (both successfully and unsuccessfully). Some have pointed out that the black bloc people were taking a different route than the main march, so the leaders among the legitimate demonstrators wouldn't have been in the areas. Some have pointed out that the black bloc were simply vandalizing property, not putting anyone's life and safety at risk, and if you call 911 to report a property crime in progress the operator will tell you to stay out of the way and keep yourself safe.

My first thought, as I mentioned in #9 of my braindump, was that not everyone can make people listen to them. People tend not to listen to me, so I normally couldn't stop them no matter how hard I tried.

But then, while reading James Bow, it occurred to me that in real life, in the regular adult world, there is no "Why didn't you stop him?" We are held responsible for our own actions; the people next to us aren't held responsible (directly or indirectly) for our actions. I can best explain with a couple of real-life examples.

Example 1: when we met Eddie Izzard, I went babbly stupid and made a complete ass of myself. Everyone else in our group was cool, but my idiocy reflected poorly on our entire group, thus depriving everyone of whatever awesomeness happens when a celebrity thinks you're cool. If Poodle's cool friends weren't far too polite to say anything, they might have said to me "WTF are you being such an idiot for?" However, they wouldn't have said to, say, TravelMaus (who was adjacent to but unaffiliated with our group) "WTF are you letting her be such an idiot for?" They wouldn't even have said that to Poodle, who was the one who brought me to the stage door in the first place. They might have asked him in a private moment "So what's her deal? Is she completely socially inept?" But it wouldn't have occurred to anyone to hold someone else responsible for not stopping me from being an idiot. My idiocy is on me, even when it ruins things for everyone.

Example 2: once one of my co-workers tried to bring his new dog into the office to show it off. The building security guards stopped him and told him dogs aren't allowed in the building. I repeat: they stopped him, the man with the dog. They didn't stop the people walking into the building next to him and tell them not to let him bring the dog in. So then he hung out with the dog outside the building, and some of us came outside to meet the dog. We walked right out the front door and then petted and played with the dog in full view of the security guards. We were quite clearly pro-dog and anyone with half a brain could see that we had probably encouraged the dog infiltration. But they didn't do anything because playing with a dog outside the building is totally allowed. They didn't scold us for wanting the dog in the building, they scolded the person who actually brought the dog into the building. Because that's how the world works. If you do something that's not allowed, you aren't absolved of your responsibility just because there are people doing nothing to stop you, and perhaps even watching with interest.

So why should the bad guys get a bye?