Copyright law is a weird thing - it's certainly been skewed well beyond its original purpose, and the fact that there's stuff that's 100 years old that is not yet in the public domain is... well, wrong.
But computer games that are 20-30 years old are still well within a reasonable timeframe for copyright protection, but, as with movies, books, music and more, it's good if the media is *available* - either copyright holders should allow it to be sold or licensed, or they should release older stuff for free.
Abandonware falls in that grey area - the media (video games) are still technically under copyright, but the rights are owned by a business that no longer exists, so there's no way they can be made legitimately available for a reasonable price. Technology also comes into it. At least there's places like Good Old Games (GOG) who put in a lot of effort to make old games work on current technology and they work with rights holders so that the games can still be sold.
So yeah... it's kind of an odd place to be. That being said, Discworld 1 & 2 are available via archive.org, and I *think* that's legit.