Technically, no. From what I gather, most abandonware sites put up titles just because they are no longer supported by the original publisher. Regardless of what a publisher decides to do with it, that doesn't change the copyright laws and suddenly pronounce anything not in print as public domain. Some sites go to great pains to explain that since you can't currently buy said game, then the public has the right to just take it. Which is just not true.
On the other hand, most publishers really don't give a damn about their old work (save Amiga publishers reinventing their successes on the GBA). I doubt if you'll see too many of those clunky, CGA/EGA uglyware from the PC's "golden" DOS era being ported to the handhelds anytime soon. A few publishers outright turn down offers to allow the old games to be freely distributed, which is their prerogative. But the concept and the way it is often portrayed in the Abandonware realm, no it's not (technically) legal.