Don't Knock It

In a piece by neurologist Richard Restak in The American Scholar called "Laughter And The Brain" we get this in the final paragraph:

Humor is constantly evolving—comics’ tastes change, as does what society considers funny. Our parents and grandparents would have found this sort of joke amusing: Knock knock. Who’s there? Madame. Madame who? Madame foot’s caught in the door!  We no doubt find it juvenile and embarrassing. “Humor” based on racial and ethnic stereotypes or physical or mental disabilities is no longer acceptable, which is all to the good. However humor evolves in the future, neuroscience will attempt to explain its mechanics.

We find it "juvenile and embarrassing"?  What I see is a pretty good knock knock joke, with a decent pun not to mention a meta-reference.  Perhaps it's a bit juvenile, but it's not the least bit embarrassing.

I don't see any racial or ethnic stereotypes, or even physical or mental disabilities.  Yeah, someone's in a bit of pain (not much at that), but jokes often involve some discomfort.

I really don't get what Restak sees here, yet he's the expert on humor.  Does this joke deal with some new sort of political correctness I'm not yet aware of?