Most games don't include a notice that what you thought you bought isn't yours (especially console games). If such a notice is not included you own the copy. You don't own the copyright, but you own the copy. According to EU law (I don't know about the US) you can do anything with that copy if it doesn't violate the copyright. And, like the name says, copyright is the right to copy/control copying.
Abandonware might be technically illegal, but morally and practically there are no problems with using it. Atari won't sue you over Combat. And a crime is only bad when the damaged party choses to sue. Common sense might not be written into the law, but (at least outside of the USA) people apply it BEFORE starting a lawsuit. I think you're free to grab Combat off the internet, no matter what the exact wording in the law is.
BTW, there is a BIG difference between copyright infringement and theft, no matter what the RIAA tries to make you believe.
Theft is physically removing an object that is not yours like running into a shop, grabbing a game and walking out without paying.
Copyright infringement means creating a replica or derivative of another work. It involves a creative (not inventive!) process, e.g. writing the data to a medium, constructing a part out of a given material, etc. CI can even occur if you build something yourself (three out of my four released Q3A player models, for example). However, unlike theft, copyright infringement does not necessarily cause damage. I don't think Square lost a single cent just because I made a model of Tifa from Final Fantasy 7.