Negitoro don vs toro sushi
Technically it shouldn't be, but all the places I have been to (e.g., Japanese grocery stores or Maru Ichi in Mountain View) almost always offer not-very-fatty tuna in their negitoro don. One advantage of serving negitoro is that a chef doesn't have to trim the tuna to make it into a pretty piece for nigiri (and remove the outside parts that are equally good in taste). In other words, if you go to a reputable sushi house, a negitoro don should still be very good (and not cheap). That's why it's more common to see a negitoro maki (roll) with much less tuna instead of a full don (rice bowl).