It doesn't "give people permission" to disrespect you, but it does potentially open the door for them to do so. The majority of people are animals deep down, and they respond primarily to fear and dominance. Politeness is courteousness and civility, but it's commonly misinterpreted as weakeness and subservience by the uncivilized (the kind who will turn into the true debased animals that they are, if suddenly all law, order, and social custom disappeared overnight), and can be pounced upon by people looking to exploit that perceived weakness in order to increase their dominance with you. It's then your job to reveal your hand and show them who's boss by standing up for yourself. That's usually enough to signal them to back down and fuck off.
You can use politeness as a way to identify the unsophisticated social animals in your circles so as to not waste your time on them and take them seriously. Unfortunately, this is going to be a large percentage of the people you encounter aka normies.