As has already been discussed ad nauseum, Apple is not bein irritating to get a "30% cut" (which is a bothersome way of describing it as they don't den get 30%: they get what is left over after paying you your 70% cut and then handling their numerous fixed costs with relation to selling your products): they want control over distribution. For more information, read this entire thread.

http://news.ycombinator.com/item?id=4037065

As for how you do it, in practice 99% of licensing schemes are based either on "serial number", "mac address", "license key given to user and typed into app", or a combination of such. While some people then can't import still, almost everyone can, and that is enough to be game-changing.

Note: you don't even need to allow this list to e changed after you import the product if you are still concerned about a "30% cut". (Though, Cydia does allow this, and even fully automates it by way of a backend licensing API vendors can implement to take control of the purchasing process, and in practice vendors that start using this mostly use it as a temporary measure and eventually just switch over entirely).