Legally, ethically, there is no difference, but the one product is celebrated and the other assaulted.
It has been asserted there is both a legal and ethical difference.
I am probably out of my league with respect to legal issues, and in any case it seems very convoluted. For instance, the license (if any) you get with Rybka seems to depend on the re-distributor. I went to 3 major ones, and for none of them was I asked to click "I agree" not to reverse-engineer, etc., the product before being able to click "Confirm payment". One of the 3 had me click "I agree" about indemnity, etc., and one of the others had some boilerplate Terms and Conditions that included "no reverse-engineering", but I was never asked if I agreed to this. The validity of such "shrink-wrap" licenses is also a point of dispute [the most notable US case seems to have been decided on nuances of federal pre-emption, and was declined to be heard by the US Supreme Court]. The use of RE as a form of discovery is fairly clearly legal in the US (see the Galoob Game Genie case -- surely Galoob had to do RE on the Nintendo games to get useful codes for their product), while as I
mentioned previously, the EU in its own funky manner seems to desire to prohibit it. I don't know about the laws in the Russian Federation.
As for ethics, it seems to depend on the intent of the author. Fabien seems, at least
ex post facto to be OK with everything. As for VR, it would have been nice if (say) when Strelka first appeared he had made clear in no uncertain terms that he considered RE and re-use of "ideas" to be a violation of his rights. In reading the Rybka Forum (and perhaps I should review this all again), my impression with Strelka was more that he was: annoyed by the RE but realised it was not possible to stop (see also his general comments about RE); and that the recriminations against Strelka were that it re-used" code".
I'm not sure this is tongue in cheek (July 2007), but:
http://rybkaforum.net/cgi-bin/rybkaforu ... ?pid=20650
SG: I mean, if you put ten engineers to work a year or two decompiling Rybka, you could probably get the source code back, but at that point buying the source from Vas would probably be cheaper (and give you a much more current version, and less legal trouble).
VR: You know, actually, if somebody wants to do this, it's not so bad. It's just the Rybka team's contribution to computer chess
As I say, I wouldn't push this quotation too far, w/o first tracking down any counterpoints from that time period.