Well, here's my belly button on the subject:
While I agree with you in principle, the fact is that the s/w is "open source" and no where, that I know of anyway, is there a TOS that says or even suggests that a link back is necessary. (Agreed it's proper etiquette).
However, I remember it wasn't all that long ago that I was a N00B at b2evolution and didn't know diddly about PHP, CSS, XHTML, SKINS or who FG was. It might be very likely that I ERASED the link back, thinking that it was ugly and not knowing that I was making an egregious OpenSource s/w faux pas, just wanting to clean up my page.
So ... I don't take it quite as personally as you (and others) might when there's not a link. However, the more I help out on the boards ... the less tolerant I seem to be about it.