Palmsource didn't decide to nix multitasking from their OS... they were limited
BY LICENSE from exposing more than 1 thread to the OS from the underlying kernel (they
licensed the AMX kernel from an embedded company called KADAK in the early Palm days when they were still called 3Com, and kept with that single-thread model through 3 other kernel rewrites that they undertook).
The terms and conditions of that license specifically state that Palm may not expose the API for creating/manipulating tasks within the OS. If you need access to these calls you need to contact Kadak at (604) 734-2796, or visit their website at http://www.kadak.com So they could have easily implemented it, but their license agreement with the kernel manufacturers restricted them from doing it. The kernel itself is multithreaded, multitasking (at the hardware level), but not exposed to the OS.
Most of these "limitations" are never technical in nature.. and this is no exception.