I disagree. Assuming that it turns out that Zinio has it wrong, and RIM's content policy is indeed to prohibit only sexually explicit material as they say it is, they have plenty of justification/reason to do so. You need to think about this from the larger perspective of the actual systems in place, and not just your individual, legitimate ownership of a Playboy subscription. RIM's content policy was not dreamed up to apply solely to Zinio, but to prevent the proliferation of porn apps in App World. Contrary to your suggestion, the age you claim you are when you create your BB ID is not a verifiable authenticator of your age, since nothing prevents you from lying. (I'm just assuming that you are correct in implying that there even is a DOB associate with a BB ID.) Of course RIM's interest here is in protecting itself against lawsuits not just from parents angry that RIM enabled their innocent angel child access to porn, but even federal porn trafficking violations. And of course such laws vary from country to country, State to State, and even municipality to municipality -- which is compounded even more when you consider that RIM might end up being accused of delivering porn across State or international borders. Add to this the fact that Apple already has in place a similar ban on porn, which establishes a market practice, and RIM really has no choice but to implement a reasonable content restriction policy.
What you also have to understand is that RIM applies restrictions at the app level, not at the magazine-contained-within-an-app level, so Zinio had to rate itself with one blanket rating. Again the rating system rates an app, not segments of content (separate magazines) within the app. So in the end, the fact that you honestly own the content that is being blocked is unhappy collateral damage.
Again, this is all based on the assumption that RIM's content policy will turn out to be based on sexually explicit material, and not on PG-13 content as the Zinio CSR stated. IF RIM really is blocking content that tame, then there's no established practice or law which supports it, and they're opening themselves to increased customer dissatisfaction at the very least.