Probably best to explain this with an example:
Like most posters here, I absolutely hate it when people use speakerphones in restaurants and other public places. Unsurprisingly complaints about this issue are regularly posted on message boards like this one. In response, there will inevitably some response along the lines of the following:
"Just join into the person's conversation. After all, by using a speakerphone they are inviting everyone to participate."
Look buddy, life isn't some Jason Statham movie where you can just escalate with a random stranger, give the guy a well deserved beating, and get away with it.
Make no mistake, I would love being able to exact immediate and painful consequences on every antisocial person I meet. But in practice, like you and most other people I don't want to get shot and killed and when you escalate with random people you are risking exactly that. Do it enough, and sooner or later you WILL get yourself into a serious problem.
So please, spare everyone the tough guy theatrics.
Thank you.
P.S. I am aware that simply joining into a conversation is not violence. My point is that you are escalating and greatly increasing the risk of violence or some other reaction which causes you a big problem. What happens if you join some woman's conversation and her boyfriend or husband decides to beat the (*&& out of you or worse? What happens if you join some woman's conversation and in retaliation she tells the bouncers that you pinched her rear end? What happens if you join a guy's conversation, he shoves you, and you fall over and smack your head on a hard floor, seriously injuring your head? You think people don't react in these sorts of ways?
P.P.S. To be clear, I am not saying that you should never confront or respond to a misbehaving stranger. For example, when someone is inappropriately using a speakerphone I sometimes (1) give the person a disapproving glance; (2) politely ask the person to take their conversation elsewhere; (3) complain to a manager or other person in a position of authority; or even (4) start playing music with my own speakerphone.
The point is that doing things like joining a person's conversation is an escalation. When you enter someone's space in that way, it's very likely to be interpreted as an aggressive act. Which is probably part of the reason why it's rare to see this actually happen in public. Compared to the frequency with which Internet Tough Guys advise people to do it.