TUAW reports that with the introduction of age-based ratings for App Store applications in iPhone OS 3.0, Apple has taken the step of preventing developers of applications carrying mature (ages 17+) ratings from issuing promo codes for the affected applications. Developers are typically allowed to distribute up to 50 promo codes for any given application release, allowing developers the flexibility to offer media free access to the applications for reviews or offer giveaways for promotional purposes.
Typically, when a developer uploads a new version of an application to the App Store, they have the ability to create up to 50 promo codes, which they can then provide to media outlets for reviewing the application, give away to users in a contest, etc. Aside from some reasonable restrictions on their usage (you can't sell them, they expire after 28 days and are one-time use only), promo codes provide quite a bit of flexibility to developers of paid apps who wish to freely distribute their app to select individuals without having to worry about the hassles of exchanging device information and doing special ad-hoc builds.
Apple's restriction on promo codes for applications rated 17+ apparently stems from the fact that parental warnings are not currently displayed when redeeming promo codes. The practice is forcing developers of such applications to consider more cumbersome methods of offering free copies of their releases, from distributing ad-hoc builds through the developer channel to offering iTunes gift cards covering the cost of the applications.
As TUAW notes, the category of applications rated 17+ extends far beyond the adult-themed applications that have gained attention, even including otherwise-harmless applications that include an embedded Web browser function or provide access to third-party data.
And if you're thinking this just applies to the massive number of adult-oriented apps that have recently poured into the App Store, you're wrong. Apple specifies that any application that may contain high levels of offensive language, violence, sexual content, or references to drugs or alcohol receive a rating of 17+. But, according to Apple, apps that feature an embedded web browser or provide access to 3rd party content also automatically require the 17+ rating, regardless of the application's content or intended audience.
