> $99/year developer membership

Yeah...

> then why not use a Raspberry Pi or something?

Because a Raspberry Pi would be very inconvenient to use as a smartphone, obviously. Doesn't explain why it makes sense to treat one ARM computer as a mildly locked jail (Macs) and another as a hard locked jail (iPhones and other handhelds); I feel like the only explanation here is that the smartphone duopoly has worked hard to condition customers into accepting this.

> I feel like the only explanation here is that the smartphone duopoly has worked hard to condition customers into accepting this.

This is older than the current smartphone duopoly; IIRC, with a few exceptions like the N900 and the Palm Treo, cell phones were always very locked down. It probably came initially from the phone companies being control freaks, and wanting to restrict which devices (even mechanical ones with no electrical or radio interface) could be attached to their phone system.