I'm all for volunteering and collaborating. It's a great way to network. But there is a very thin line between volunteering to help out and being used as free labor. It's so commonplace, many now-professionals even stress going the extra mile on these "free" positions in order to keep your foot in the door. As the practice spreads in other industries due to the economy, unemployed workers and the government are starting to question the legality of the so-called "adult internship."
Just as both articles point out, working for free for the direct financial benefit of a company is illegal. The "non-college intern" PA position on professional shoots has to stop. If the film cannot budget some type of compensation for you, then it probably will not help you advance your career. Be friendly and open to assignments, but be mindful.