I've wondered the same. If you're in North America, then legally speaking it seems to be fine... since you're only copying the design and not mathematical/programmatical font data, and font designs themselves are not protected by copyright law. (In other countries/regions you should definitely check what the IP laws there say about type designs).
Whether or not it's ethical is getting into a rather murkier area. Inasmuch as I've been able to sense any kind of general consensus amongst type designers, it seems to be that type design really ought to be copyright-able even if it isn't; and so I get the impression that there's a kind of "gentleman's understanding" (apologies to the ladies among us) that the proper thing is to behave as if they were.
For myself, at least for now I've tentatively decided to adopt the approach that a design is fair game for revival or reinterpretation if it would be out of copyright under local law (in my case, Canadian law) if it actually were copyright-able.