The hope is that you'd buy the Apple-branded one. Obviously Bluetooth is an open standard -- but that was the business intent.

An adapter is less convenient to use than having the Headphone jack on the device.

Is it really that farfetched to think they removed the Headphone jack to increase profits? Clearly, I'm arguing for it, but if this absolutely out there, I'd like to be brought back more sensible thought processes.

Not even the included earbuds with an iPhone purchase come with a 3.5mm jack. It's Apple's proprietary port, so you can only use Apple's wired earbuds with Apple products.