I think the link (now) points to the wrong line in the source code. I found it, though.

Anyone understands why they do this?

GNU Parallel's citations was always weird - the author IMHO goes too far, in requiring each user to type "will cite" first time they use the app, even if context is entirely non-academic. Of course the author has the right to ask for anything, but I think that's too much, plus parallel is not nearly complex compared to other tools people use, like compilers or math libraries or Linux kernel - and those do not need citations.

I suspect that the main reason people are using of "parallel" is because it was the first to grab this very nice, very obvious package name. "rush" and "gargs" and "pexec" simply does not have the same obviousness to it.

If you think asking for citations is too much (and I do), I recommend using something else. Yes, "parallel" does have some nice features, and something you might need to use an extra command or two, but it is worth it for the peace of mind, plus it respects author's wishes too.