The big problem on tablet devices is constraints in memory. Yes the unix way is to have various small utilities work together through pipes and interprocess communications. The big challenge at that point is how to do you manage memory? What if a single process in that huge chain uses up all of the memory and another process gets killed by the OS which was part of that chain (do note, no swap on mobile devices)... How is the calling program supposed to handle one of its subprocesses suddenly dying?
Do note that the same is true on Google's Android OS. You can run as many threads as you want, but no calling other executables to do work in the background ...
A current tablet device, even a cheap one, has about 256Mb of main memory. Android (at least the cyanogen mod kernel) has also the ability to use a part of the main memory to store compressed swap partition data. That leaves about 80 Megabytes to userspace programs (on Android). About the amount a normal desktop computer had in 1998.
LaTex dates back to the beginning of the 80s. Given that it has the ability to create large documents, I doubt that the required amount of main memory scales linearly with the size of the document.
Also, when you use subprocesses, besides checking their return value, you have some sort of interprocess communication which should detect when one process failed to finish his job.
It looks more like the smartphone runtimes and their sandboxes just don't support it at all, because modern GUI applications rarely build upon multiple executables in the background.
Android does have IPC mechanisms (Intents and remote methods) that can be used to have another process do work in the background. The difference is that even if you use them, you're still not likely to wind up with something that looks like a set of cooperating command-line utilities.
In any case, I absolutely agree that resource constraints aren't keeping LaTeX off of our mobile devices.
I have TeX Live available in the Ubuntu chroot on my HP TouchPad, so you're right that it's not a resource thing. Admittedly in this case webOS provides a pretty standard Linux environment under the hood, so it falls pretty firmly outside the Android model (much less iOS).
Do note that the same is true on Google's Android OS. You can run as many threads as you want, but no calling other executables to do work in the background ...