By taking out a bill board, he was making it hard on parents, upsetting little children, and calling attention to himself (he did get his name in the paper, didn't he?).
Regardless of the rightness (or, wrongness, in my opinion) his methods are what are objectionable. I would point out that while small children may on occasion be disillusioned by discovering that Santa is a myth, a whole lot of people are going to form the opinion that Christians are miserable fanaticts who haven't an ounce of fun in their being. Not a very good missionary tactic, in my opinion.
Extremely well-put!