hedwards: Not any more so than any of the games they have on the site currently. Which is to say not at all.
Navagon: If the games contain proprietary code to get them working on those platforms then it could be illegal to sell them as emulated titles.
The reason why console manufactures might act is because it would mean that the console manufacturer is not getting a cut of the profits, which is something upon which console manufacturers rely to see a return on their investment.
Naturally if these titles were ported over then it wouldn't be a problem. But emulators and the games themselves often contain code which renders them illegal without consent.
I disagree, there's absolutely nothing illegal about emulation, at least in the US, provided that it's a clean room implementation. Nintendo might throw a hissy fit about it, but I doubt very much that they have any right to prevent Activision from signing a licensing agreement with GOG for 20 year old games.
As for the legality, the patents on the NES, SNES, Genesis and anything older have long expired, at least in the US, as far as copyright goes, it really doesn't apply. That is unless the emulator developers got hold of official source which most likely doesn't even exist at this stage. And even that is iffy as you can't copyright headers and that's the bit that Nintendo is likely to throw a hissy fit about.
Also, I'm pretty sure that there have been legitimate rereleases that used ROMs in the recent past and I have yet to hear about any lawsuits covering it. Fact is that it's unlikely that a court would be impressed with Nintendo or Sega claiming to still own exclusive rights to a game that hasn't been sold new in probably close to 20 years.