99% of the computing activity that goes on daily is information and media consumption, not creation. First and foremost, we surf the web and, along the way, watch photos or various media clips that we encounter. Gone are the days when a computer meant something used for word processing or writing a spreadsheet. While we still do those things in similar absolute amounts, the sheer number of *other* things we use our computers for has dwarfed these activities to the point where there is a large market for a machine optimized primarily for consumption. The netbook was aimed at this catagory, hence its name. A key to success is that such a device needs to be able to do rudimentary creation or at least editing. Again, witness the netbook. The keyboard is cramped and the screen is small, but this is fine for occasional road-warrier-style editing while mostly being used for reading email and internet surfing.
The iPad fits this niche very well, with a much slicker and more intuitive UI. Why must it be labeled and treated as a general purpose computer even though almost none of its customers would use it as such even if it were so capable? Tech journalists treat every product as if it's intended for their personal use.
As for the app store, Apple's centralized control has thus far resulting in only a literal handful of highly-desired apps being rejected and not made available. A handful out of more than 100K. This is an exceedingly small fraction, with almost zero impact on how anyone has used their iPhones and iPod Touches unless Google Voice is your raison d'etre. Improving usability by a few percent while opening up the floodgates to a far larger share of malware might not actually be the right tradeoff for the majority of customers who, again, are using the device for consumption and will likely never even buy more than a handful of apps to supplement the build in apps.
There's no slippery slope here because the same model wouldn't be tolerated on a Mac. Once those who'd be better served with an iPad are taken out of the equation, the remaining Mac customers buy these machines because they actually create content, need choice and see a net benefit to having control vs. having to search many stores for the app they need and police for malware. Indeed, since the software used for most purposes tends to have already been largely standardized (Office, iLife, Photoshop, Final Cut, etc.), most users don't actually even exercise that much choice at this mature point in the product lifecycle, tending to prefer to stay compatible with the knowledgebase that exists for these dominant products.