I have no issue with people saying, "Don't go to the theater, I think it is a bad influence" or "Since I quit going to the theater, I have been so blessed". (and go into why it is a bad influence, or why you are blessed)
But saying "You are sinning when you go to the theater" or "Do you know who I saw coming from the theater the other day, Jon. He is such a sinner" or even thinking "Oh, that Jon, such a sinner for going to the theater" is not good, and can even be harmful.
Theater going is in no wise a sin if we look at the Bible. It can only be maybe thought of a sin if you add a lot to the Bible. It is church tradition that going to the theater isn't good, this is fine. Just like it has been church tradition that caffiene isn't good (it seems to me that this is as ignored as theater going).
Adventists are prone to legalism in the past, we should make sure that we are still not prone to legalism now. We also need to watch ourselves that we do not copy the actions of the pharisees in the past, nor that we drive people from the church.
JM