The secret is that Apple did all the groundwork years in advance, as part of rolling out the Apple Stores, the iTunes Music Store, et cetera.

But in many countries, such as Singapore, iTunes stores (music, movies, etc) wasn't available. They still aren't available, but the app store is, once they rolled out iPhones worldwide in a matter of months.

Doesn't Google already have a way to remit money to adsense partners across the world?

Presumably. But AdSense is a rather different business.

Consider: I don't think advertising is subject to sales tax. Here's New York State on the subject:

http://nystax.custhelp.com/cgi-bin/nystax.cfg/php/enduser/st...

When you go into a product business you may have a different set of legal issues to consider. It's also B2C rather than B2B, which changes the sort of customer service you need and (I'm guessing) dramatically increases your vulnerability to things like chargebacks and stolen credit cards.