For my $0.02 in regards to the "legality" of old/outdated software such as DOS... it's a grey area. Is there copyright involved? Certainly! Does a company have a right to protect their IP? Absolutely! Is it right that a company lets outdated software die rather than release it as a downloadable freebie, to the public domain, or absolutely worst of all, stop offering that software, even for a fee, when there is still a need for it, however small? To my mind, YES - that is where the real crime is at.
I realize I speak from a user's point of view rather than from the aspect of a software developer or a company with IP to protect.. but I believe in the "abandonware" concept: if/when a company abandons their software, then after a 7+ year time-frame, to me, it's fair game for not-for-profit distribution. To me, MS and PC DOS fit in that category.
Although in the case of DOS, it's absolutely fantastic that there are freely-available, perfectly functional alternatives. And as has been said... OS's are cheap on the used/auction market.