Did you check the index for the entries of people with missing surnames? RootsMagic, like every other genealogical program, would surely be making unwarranted -- and unwanted -- assumptions by assigning surnames automatically.
The father probably showed up on the pedigree screen as well.
I grant that single names are a problem; there are a few Native Americans in my wife's tree and I've entered their names as surnames. There may be other and better ways of dealing with those names, but I wouldn't expect much help or consistency from Ancestry trees in that regard.