I agree that the in app purchase system is flawed and annoying for everyone. But I'm pretty sure developers can build trials natively. or am I not understanding your point?
They can now, but couldn't for a loooong time. The damage is done, and everyone now just expects lower prices for iOS apps without realizing that they are (subconsciously, perhaps) pricing in the "it might suck" insurance when they no longer really need to.