I don't know much about tax law, but I would say the main reason besides assurance to the donors is for tax purposes. I'd be surprised if most fan-films accepting donations on that level didn't file for that tax-exempt status. I wonder if STNV and Renegades filed for it? Like I said, I don't know much about it and maybe there are other exemption statuses to get around it.
I'm a CPA and I work with a number of nonprofits. There are a few issues that a fan-film would have to deal with if they pursued 501(c)(3) status.
First, you can't file for the status and receive it automatically just because you don't intend to run your organization as a for-profit enterprise. You have to have a legitimate charitable purpose as defined under the statute. So the first hurdle is convincing the IRS that your fan-film carries out some sort of educational or other charitable purpose and qualifies for 501(c)(3) treatment.
Second, if you get preliminary approval and can begin accepting charitable donations, you have to comply with the laws of the state or states you will be soliciting donations from. This typically requires some sort of registration with a regulatory body (attorney general or secretary of state) and annual filings.
When you start receiving donations, you have to provide contemporaneous acknowledgement to your donors that the funds they are giving you were a donation and no goods and services were received in exchange. If you are giving goods and services as part of your donation packages (which these fan-films do) you have to disclose the fair market value of such items to the donor so that they are aware that the amount they claim as a donation has to be reduced by the value of goods received. If you donated to a fan-film and did not receive this type of statement, they are unlikely to be a non-profit.
You are also required to file Form 990, the annual information return for nonprofits. This is a public document. You can look at any organization's most recently available Form 990 at guidestar.org. You can also request it directly from the non-profit, although they are allowed by law to charge you a reproduction fee.
That's just the tip of the iceberg, but it seems like a lot for a fan-film to have to deal with.