That still means that I must offer the entire source code for my TCP stack to the world. That's not how any other free software license works - if I'm running a GPL'd TCP stack like Linux's, and I've made modifications to it, I don't have to offer the sources for it unless I'm distributing the program. Merely using the modified code to be on the network does not imply a source obligation.

I mean, it is probably reasonable, in the abstract, to have licenses that say "You may not modify this software internally and use it at all unless you make the changes public." Plenty of licenses have required that. Those licenses are non-free in the FSF sense, and they have rejected such licenses before.