I think the reason is user experience. Future computers
will be "theme parks", because non-geeks don't understand
the proper limitations of hardware.<p>For example, 99% of annoyances of computer users is that
they install too much software for the resources of the
machine they have. Then the computer starts suddenly working
slower and slower -- and they don't know why. This happens
with Macs and Windows.<p>In a machine like the iPad Apple can control what software
will run and make sure that it is adequate to the resources
of the machine. Also, from what I see, they limit the
possibility of concurrent running programs to interfere with
the user experience.