I'm also not sure how legal that is, since it's essentially the same sort of thing as phone-tapping. I suspect if it becomes more widespread, there'll end up being lawsuits and it'll eventually be repealed.
But either way, abandonware is indeed technically legal, since a piece of software isn't "abandonware" unless and until all rights to that piece of software have completely lapsed. And if all rights have lapsed, that piece of software is technically public domain.
...At least, as far as I'm aware, anyway.