personally I would have welded the inside if I was only welding one side because I would not want a place where little infections can start.
I do understand why he did not, he wanted you to see a good weld, and you do till you look inside.
He did not do both inside and outside because you did not ask him to
the inside would be a lot more difficult to do.
if he will do it for free have him touch up the inside. His work looks good and it has the proper heat and travel to get good penetration.
remember, welding around the outside of a pipe is not easy
Can you explain the best way to weld the inside face? I just don't see how you can do it when the access into the keg is a 12" hole - getting both hands in there for the torch + filler and also being able to see what you are doing (including underneath the fitting) doesn't seem likely?