The rule is “officially free to play”. The Infocom games are easy to find unofficially, but they have never been made officially free by Activision (aside from certain promotional releases with unclear licensing).
I’ll agree with you on one, though: Hitchhiker’s Guide to the Galaxy (#87 on the IFDB Top 100) has been officially available for free on Douglas Adams’ website since 1999, and from the BBC since 2004.