I think it's normal for the publisher to hold those rights (perhaps shared with the original authors, depending on the details of their agreements), so possibly all that would have been required here would be for the publisher to approve doing this.

Or maybe Rucker and all of the other authors are friends, and keep in touch, and he just literally called all of the up and said "Hey, can I post Mirrorshades online for posterity?" and they all agreed. Who knows?