Want to read the source? There are a few decompilers out there that you can use on the story file that'll help.

Want to modify someone else's game? I would say that decision is up to the individual authors themselves, but the general consensus seems to be "not bloody likely." Or maybe even just "write your own damn game."

And for good reason, look -- one-half of the term interactive fiction is the word "fiction." Good IF games are not just bits of code, but good stories. Stories that an author has worked long and hard to plot and write, stories involving characters that an author has had to develop, stories crafted and pieced together in what is often a very intensely personal project.

What, then, gives you, me, anyone the right to take an IF game someone else has written, a story that someone has personally invested so much time and energy into, and say "Well, I didn't like this part of the story, and I thought the ending should be happy, and maybe there should be some sexy parts, so I'll change it to something better"? Do you think the author would be happy that you did? Would you dare do the same to your favorite novel, comic book, or screenplay? Do you think people would care to read your version? (Well, hell, they wouldn't have to care, you could just put it up on the Internet and call it "fanfic", a lot of kids already do.)

Or would you rather just take some ideas and characters of your own and write your own game? Go ahead and do that, and GPL it while you're at it, and be happy with it and what people do to it. There could very well be a nice collaborative project that'd come out of such a thing, but for the most part, IF writing is a very personal venture. And let's face it: not every piece of code needs to be open-source. I may be the author of a few silly frippery one-joke games, but the work is my own and if I felt they'd needed third-party rewrites, I'd ask for them.