Well, trying to legitimize their actions is just a load of bologna. The disclaimers on these sites say things such as "It is legal to download this game if the company has gone out of business" or "You are authorized to download one copy to replace your own if you lost or broke yours". I remember claims on rom servers saying "You can download roms for 24 hours before being required to delete them". The hell? They're just confusing the ones who don't want to break any laws, and the real pirates are pretty much going to ignore it anyway. It's just wishful thinking and vain attempts to avoid lawsuits.
How many games would have been lost to the cosmos if someone wasn't keeping a backup copy of it somewhere? You could do a simple search for "roms" or "abandonware" and see huge lists of games you probably never even heard of. Now, if you were saving these games for posterity, that'd be a commendable effort. You'll be praised and lauded by others who appreciate the free exchange and preservation of data. It's like being the Internet version of Robin Hood or something. But these sites basically operate on the premise of, who's going to stop us? I'm not an accountant or lawyer, but I would be willing to guess that with these games being thrown around in disregard of copyright laws, it basically means that your copyright is worth jack shit. Why do we even have copyright laws then? Why do they have a 100 year lifespan and not just die with the company? (This is a whole different beast of a discussion though)
Maybe people who download abandonware consider themselves connoisseurs or historians or anti-copyright activists. Just realize that you're indulging in a guilty pleasure.
HotU, one of the best abandonware sites (though now, regrettably, dead), contains no such disclaimers, only hosts files if the game could not be found in online stores and removes titles immediately if a copyright owner or their representative requests it; clearly you use a broad brush. In practical terms copyright
is 'worth jack shit', as is money, law and any other social construct - but, as you say, that's another can of worms. There is no legal basis on which 'abandonware' is legitimate, but do you honestly believe that enforcing arbitrary restrictions from which none gain is sensible? That the creators of the games would prefer them to gather dust than be appreciated? I have no pretentions of anarchism or connoisseurship, I'm simply pointing out that you conflate legality and morality, and so I remind you that abandonware is not a guilty pleasure, merely an illegal one

No other form of piracy, to my mind, is in a similar position.