Why does Apple call USB-C Thunderbolt?

Look at any specs for a new MacBook and it says it has Thunderbolt ports. So if you buy a Firewire-Thunderbolt cable you'd expect it to work right? WRONG. The MacBook has USB-C ports, so why do Apple persist in calling them Thunderbolt, it just confuses everyone. When I returned the Firewire-"Thunderbolt" cable to an Apple store the guy said they get heaps of returns on stuff that's supposed to go into a Thunderbolt port but doesn't because it's a USB-C port.