It is, but you're given a table, not just told to sit anywhere. I think people are assigned a table from the computer, so that waiter stations can be balanced to avoid overloading. (Or am I remembering that from HAL?)
One of the instances quoted above was dinner. With some tables being open seating, people may assume that being seated somewhere doesn't mean they have to stay there. But even so, it's rude and entitled. Who does that in a shoreside restaurant where they're shown to a table?