Hate to advocate for the devil but, as somebody involved in fanart, people want objects for the media properties they love and if they can't get them legitimately they will make their own or get them from somebody who didn't license the property. And if Waterson didn't spend his own time and money sicing lawyers on the latter his syndicate wasn't going to do it for him if he didn't license it.
You are correct. Also, copyrights do expire eventually (takes longer than it should, but they still aren't perpetual)
Then there's the fact that anyone who wants more C&H comics can now just ask for them. I hate to think about how Watterson will feel the first time he sees that happen.