Different publishers and developers have different attitudes toward Classic Gaming, and the issues of emulation, royalties, intellectual property, etc.
David Braben and Ian Bell, the creators of Elite, have effectively put that game into the public domain as they no longer make any profit from it. Sega have a surprisingly lenient attitude to emulation. They seem to see it as no threat provided that only games that are no longer commercially available are used. They have even benefitted from the development of Megadrive emulators - when they wanted to re-release a collection of classic Megadrive games on the PC, they simply hired the coder of the most complete freeware emulator, saving themselves the effort.
Activision hold onto their own properties very tightly, as they have shrewdly stockpiled a number of classic titles and franchises which they can rerelease and port at their discretion. Sony and Nintendo are dementedly harsh over copyright, as befits a company that has a seat in the RIAA and MPAA and one that makes money out of 11-year-old hardware.
There are two small 'loopholes' that emulation enthusiasts use to ensure their legality : to only keep the game ROM for a 24 hour 'evaluation' period, or to own a copy of the original game cartridge / tape. As a rule of thumb, I think that if a game is no longer commercially available (and in many cases the hardware itself may be hard to track down), you should be allowed to play it on an emulator.
Although I am all for people playing old games as a bit of harmless entertainment / art appreciation, I get pissed off to see warez kiddies peddling rafts of recent PC titles under the misnomer of Abandonware.