Watterson intentionally gave his Calvin copyright to the public domain.
I'm fairly confident that this statement is, in fact, untrue.
The original copyright is owned, of course, by United Press Syndicate under the usual terms.
The reproduction rights for the various publications are owed by Andrews McNeilly, and Waterson himself still owns the rights to the characters. As recently as 2006, Dartmouth College was forced to rename the 96th Winter Carnival as "Stupendous Games: Mischief in the Snow" because using the original Calvin and Hobbes theme would have violated Waterson's copyright.
I have been unable to find any source that suggests that Waterson placed the characters in the public domain, and an number of sites that state otherwise.
Don't like it? Go piss up a rope. or a W. Or make up yopu own Calvin cartoon, of Calvin pissing on a Calvin pissing on a Calvin... Or complain to a moderator. Ooops, can I say pissing? whether or not somebody can have a cartoon of it?