Snoopy's appearance has been gradually progressing all this time, but this strip seems like one of the more obvious changes.
The later jokes of Snoopy acting a lot like a person, and being treated like one, evolved out of gags such as this one. When Snoopy has been half-human for twenty years it is hard to get humor out of his doing things not expected of dogs, or being treated as human.
Really, what could the girls be so defensive about? They're, what, six?
