I'm sorry but I think you're mistaken. There's nothing in the default theme that encodes ampersands in post titles. At least nothing I can find. From what I recall, there's very little filtering applied to
the_title apart from stripping out any inserted markup.
I don't know the WP-Max theme but is it by any chance using a Strict
DOCTYPE. The default theme uses Transitional. If you validate the same content with a Strict
DOCTYPE and then repeat the process with a Transitional
DOCTYPE, you can get far more errors reported in the former pass than in the latter. The "Strict" term is a clue as to why.
I wouldn't worry too much about the odd unencoded ampersand. By themselves they won't do any major harm. The biggest problem is that, if you have a lot of them, they can create enough noise to hide serious issues when you validate your blog.