PBS Joy of Painting's Bob Ross did not actually create any of his paintings spontaneously while the show aired: Was this deception fair to the viewers?
Yes it was fair to his viewers.
Painting is a art form that only a small handful of people can do well. Bob Ross is one of those people showed himself painting the pictures. Although he had practice painting them beforehand he still had to paint it live and it's still a talent. Even with practice most people can't do what he did.
Yes, Bob Ross's use of a reference ensured that his show would be worth watching.
Yes, Bob Ross used a reference while he was painting, but he was still the person who had painted the reference in the first place. While the show exaggerated his skills somewhat, it also ensured that each episode would be an enjoyable experience for the viewer by making sure that there would be a minimal chance of a flub.
Deception was fair
I think that it almost goes without saying nowadays that any so-called "reality" television show is far more scripted than the producers advertise. Although this was not as prevalent when PBS' Joy of Painting was being shown it is still a fair assumption to make and I think that any deception is small enough to forgive in this instance.
Painter Bob Ross was not that deceptive to his viewers
If anyone thinks that Bob Ross was deceptive while creating paintings, they have issues that go beyond merely painting. The whole point of the show what's to watch a creative painter make a painting. Who cares if it was spontaneous or not? The man was talented and created beautiful works of art.