It's hosted on one of the author's sites. The collection itself is (as far as I can tell) out of print. It's falling through the cracks of "too complicated for a publisher to figure the rights out of" and "not lucrative enough for anyone to care".
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?