Politeness isn’t an inherently good or bad thing. Impoliteness isn’t an inherently good or bad thing. They’re both communicative tactics.
1) The politeness tactic is, “I’ll use these established culturally-specific modes of conversation. That make (default) people less uncomfortable about the method of my communication.”
In non-confrontational situations involving strangers, this is generally a way to get through things relatively easily.
In confrontational situations, it makes things easier primarily for the person in power, the person who counts as more ‘default’. The comfort level of the less powerful is irrelevant to politeness, and the difficulty for cultural outsiders tends to be ignored.
And it’s a method, it doesn’t determine content. It’s possible to politely be a horrible human being, and many people who do this claim that their politeness excuses/justifies their “opinion”. Often about whether other people have a right to bodily autonomy.
2) The impoliteness tactic is, “I’ll disrupt these established culturally-specific modes of conversation.”
This can have a couple of different effects. One is to make people uncomfortable (particularly people who cling to politeness, probably because it benefits them). Another is, if multiple people are violating norms together, that’s a bonding thing. These can happen simultaneously, where more than two people are involved in the conversation.
Swearing is often a way to express impoliteness. It’s also often a way to express emotion - which is often itself considered impoliteness, because politeness is primarily about/for the comfort of the privileged.
So yeah, in non-confrontational situations, being randomly rude can be mean or nasty. But in confrontational situations it is often the best way to upset a power balance.
Which is why we can tell, the moment someone starts tone-policing or otherwise attempting to squish impoliteness or dismiss people for being impolite, that this person was previously comfortable, is now uncomfortable, and doesn’t like it and wants to shut everyone else up so they can go back to their nice polite privilege.
And the only people who can do that and still be considered polite? are the people in power.