It probably is intentional. You don't mention what type of Pirate you are concerned about. My max Witchdoctor has about five companions that can learn Relentless. Only three Companion types (Swashbuckler, Privateer, and Buccaneer) could use Relentless. The ones I have found on all Pirates are El Toro, Ratbeard, Catbeard, Kan Po, Milo Graytail, Ridolfo Capoferro, Black Angus, Peter Quint, Samoorai Champion, Kobe Yojimbo, Thunder Hoof, The Marchioness, Egg Shen, Tonka, Monterey Jack, Eric Jolly, Fan Flanders, Subodai, Black Storm Raider, Yakooza Enforcer, Toreador Maestro, Tyson, Wing Chun, Lucky Jack Russell, Masamune Goto, and Black Storm Elder. There are probably more since my Pirates are not all max or at the end (so far) of the adventure. On the average, there are about 4 Companions per Pirate with the Relentless talent. I think this is very balanced since if every fighter had Relentless it would be overpowering. Also I shudder to think about the what would happen if all our melee enemies had Relentless since what we get they get.
It doesn't seem to be class related since that list includes Buccaneers, Swashbucklers, and Privateers. I don't think that Musketteers or Witchdoctors will ever get Relentless since they have their own multiple attack Talents.