imagine you want to go from city A to city B
normally you would search for tickets from A-B
however, during your search, you would NOT see tickets that go from A-B-C.
why is that relevant? sometimes tickets from A-B-C are cheaper than A-B. so you would buy a ticket (A-B-C) and simply leave the airport of city B (not boarding the B-C flight).
the website in question made it easier to find these types of tickets. These tickets are commonly known as "hidden-city" tickets.
the airlines would prefer you to spend more money on an A-B flight rather than buying the cheaper A-B-C ticket.
they are now suing this website to enforce their preference on consumer choice.
if there were something illegal going on, it would be limited to your individual contractual obligations as the owner of an airline ticket. (there could be a term that requires you to actually be present on the airplane for an A-B-C flight)
This website is simply _enabling_ people who intend to violate this (potential) term of their airline ticket agreement.