The answer to that problem is that 1866 is 2 games in 1. You can play as a Cowboy or Lawman and take the fight to the hotels, or a Mexican, Indian, or American fighting over towns and castles.
If you're playing as an outlaw, only fight lawmen and outlaws, and vice versa. You can't handle the major leagues.
It gives you a special feel of a league of power, where you're the amatuers with their pistols and bank robberies