For one thing, it isn't. I can do linux kernel dev on my desktop and laptop without voiding my warranty or risking bricking anything.
And most of the things I'd want to do with an iOS device that I'm prevented from doing wouldn't involve any kernel-level work anyway.
But more importantly that's the most pathetic justification I've ever heard. By that logic, since playing the piano is difficult we might as well sell pianos with all the keys glued together.
The truth of it is that Apple products are not designed for tinkering and they actively resist attempts to do it. To spend my time plumbing their internals to add value to a product that treats me an my kind as a sort of infection would be a pure waste. I'll do my hacking on Linux and use my iPhone for arguing with people on hackernews, it's fine.