I also want to add: There is NO such thing as "Abandonware". It is a made up term. Just because a game is no longer commercially available does not make it legal to download it.
Isn't it OK to copy games that are no longer distributed in the stores or commercially exploited?

No, the current availability of a game in stores is irrelevant to its copyright status. Unlike trademarks, copyrights are not considered abandoned if they are no longer enforced. Copyrights do not enter the public domain just because they are no longer commercially exploited or widely available. Therefore, the copyrights of games are valid even if the games are not found on store shelves, and copying or distributing those games is a copyright infringement.

Haven't the copyrights for old games (like Atari & Commodore) expired?

U.S. copyright laws state that copyrights owned by corporations are valid for 75 years from the date of first publication. Because video & computer games have been around for less than three decades, the copyrights of all video and computer programs will not expire for many decades to come.
For more information, please check the IDSA's website: http://www.idsa.com/piracy.html