I'm pretty sure it is Cabotage but I don't think its illegal for you to FLY, just illegal for them to offer it.

From Wikipedia
Cabotage situations can also occur as a consequence of hub-and-spoke operations. Consider that Air Canada has a major hub at Toronto that offers flights to several U.S. cities. While a passenger is able to buy a ticket from Boston to Toronto, and a separate ticket from Toronto to Seattle later that same day, both flights cannot be offered on the same itinerary because this would effectively be a U.S. domestic service