This happens all the time. This is nothing new. A lot of things are prepared ahead of time, and even prepared by other people so as to save time and make the filming go smoothly. So what if he wasn't actually painting during the episodes? It was still enjoyable to watch him and listen to him as he 'painted.'
I am of the opinion that PBS should not need to apologize for the fact that Bob Ross's paintings not being originally done during the filming. What matters is that Bob Ross did do the paintings himself, whether or not it was done during the filming is inconsequential to the originality of the paintings.
No, PBS should not make a public apology since it has been discovered that Bob Ross's paintings were not originally done during the filming. Even if they weren't done during the filming process, those paintings are still by Bob Ross. Yes, it was wrong for PBS to make people believe something that wasn't true, but a public apology is not necessary.
No, PBS should not make a public apology since the discovery that Bob Ross's paintings were not originally done during filming. They had no idea how Bob Ross managed his business. If they were not aware, then they do not have a basis for an apology. Apologizing for Ross's manipulation of his paintings could open the door for additional apologies for other actions from individuals. They would be better off not saying anything and not adding to any potential scandal. Ross should be the only one held accountable for his actions.