IB does not appear to be suing over the forking itself. They acknowledge in the complaint that the content is under CC-by-SA and that this allows people to copy it.
They complaint is basically about the name Wikitravel. They claim trademark on the name and are upset that the forkers are using the name to promote the fork in a way IB feels misuses their trademark. They also throw in unfair competition and a couple other things.
At no time did we even consider using the name "Wikitravel" for the fork. On the forking email list, a recurrent problem has been participants suggesting a new name. The plan was do deal with names last. Now that we are moving to Wikivoyage first, a cleanup pass is being performed to remove "Wikitravel" from all text and maps.
IB is just throwing stuff at the wall to see what might stick.
They complaint is basically about the name Wikitravel. They claim trademark on the name and are upset that the forkers are using the name to promote the fork in a way IB feels misuses their trademark. They also throw in unfair competition and a couple other things.