The fact that this needs to be broken down and explained to you is a testament to your downright stupidity.
The point, oh belligerent and uncontrollably flatulent one, is that you wouldn't say such a thing to a person's face, (stranger or not) during an altercation, because it's totally inappropriate. You see, among civilised (White) people, there are certain types of social etiquette and unspoken rules which we follow. Mocking another person's dead relatives for example, even during a heated argument, is completely inexcusable and would likely result in you being ostracised and shunned for being not only disrespectful, but potentially deranged also.
Is this a cultural thing or just you being you? I highly doubt Flea would ever mock someone's dead relatives, so I suspect the latter to be the case.