Guests should not presume they can sit where they like. It’s manners to ask someone if this seat is usually occupied by a regular. I think it’s more of an issue for men than women, but I know my husband gets upset if people just sit in his place . He doesn’t ask them to move, though, but he thinks it’s disrespectful, especially if the newcomer is a guest of the regular neighboring occupant.
The only time I think it’s appropriate to volunteer to give up a seat is for a Bar Mitzvah or Aufruf. The family & guests should sit “eibernun” and I always give up my regular seat willingly. I offer it. But on RH & YK, I have a reserved seat & I expect it to be empty when I get there.