Isn't zSnes largely x86 Assembler too? That's why its s damn fast. I think I'd be quoting from their main page if I said "Zsnes is an emulator for the super famicom home entertainment system in ASM and C". Could have misphrased it slightly, but its that sort of thing. Which means lots and lots of nasty ASM to ASM translations. Which wouldn't be fun, methinks.
Hmm, I doubt everyone would just claim it as their own if he released the source. At least, not insofar as just changing the menu and re-releasing. It'd be done on the "Suchandsuch has fiddled with Intellecto's Snes9xGP to switch the directory to \snes\" or somesuch. So far, people just don't seem to be into nicking others' work here. Well, some seem to be into warezing (and we all hate them for it), but not claiming work as their own...