I have maybe a more liberal view on this than many do.
For open source programs, I don't really understand very much how someone can distribute a program as open source and then be upset if a derivative shows up that uses some of its concepts, ideas, or (if allowed by the license) even code. To me, the relevant question to ask about such a derived program is, is it violating a copyright or license? If not, then it's a reasonable use of publicly available source code. In fact, I am surprised that we've not had more such derivatives, given that a number of very strong programs are available now under GPL.
However, Strelka, which is apparently derived from a closed source program, is not ok in my book. It's a different case because the original author did not intend to make his work public.
