the thing is the rules with pipe smoking always kind of remind me of lies we commonly tell children. Good intentions but not totally true.That's actually untrue. The mortise swells and contracts through heat and moisture while smoking. The fear of removing a stem from a warm pipe is the thought that a tight fitting stem could snap the shank if an attempt is made to remove it. I've always removed stems from a pipe immediately after smoking to clean them and no damage after thirty years.
Example you should wait for the pipe to cool if you are having a struggle getting the stem out. But instead of telling us that it's let the pipe cool before taking the stem out. I think the pipe council knew we'd fudge it so they just tell us never do it in the hopes we sorta listen.