The morale bonus doesn't seem to work for human troops. Later in the game when I made an army of Dwarves they got the +1 morale for being all one race. I didn't even have to fill all the army slots to get it.
There are artifacts that will raise the morale for certain troops as well, such as a keg of beer that gives +1 morale to Dwarves. Personally, I think it should give +1 morale to everyone.