Games was a much smaller world back then, and pretty much everyone with a 8-bit system had tried their hand at both writing games and hacking other peoples. When you write assembly language yourself, and own a debugger, then everything is "open source".
I cracked a few games, looked at someone else's code but I can assure you that at the ASM level, it's very hard to understand that phong-shading routine to the point you can actually re-code it yourself...
Depends on the game. Some games had so few comments that they looked pretty much like a disassembly (Pac-Man for the Atari 800 had exactly two comments in it -- one was a copyright statement, the other was "Ha!", on the critical write-to-ROM for its rather pathetic implementation of copy protection).
