doing/not doing IO may be an implementation detail for a function. For example, say you have a function isPrime(x) that you implement using some primality test, with no IO. At some point in the future, you may notice that computing the primality test takes a long time, so instead you decide to submit the number to the server which will then return the answer. The function itself remains the same, it takes in a number and returns its primality, however the function is now asynchronous and performs IO.
Not conceptually. From the perspective of the rest of the program, isPrime(x) has no side effect regardless of which method you use to implement it. If you are working in a purely functional language, then it makes sense to have introducing IO be a major change, as it has huge potential to make a function impure, and assuming IO is a pure function is roughly the equivalent of unsafePerformIO. However, in this case, we are not even working in a pure functional language. In concept, there is no reason that implementing a function with IO should require writing code outside of that function differently than implementing it with CPU.
Then the program dies with an exception. What if a memory allocation in a Haskell program fails? It can happen just as well because allocating memory is an impure operation which you still have to perform from pure functions. Haskell just happens to pretend it has infinite memory available because forcing the programmer to attempt handle out of memory conditions would be impractical and annoying. Haskells "purity" is just a chimera, albeit a useful one. And it's just as useful being able to pretend network operations are pure too.
generally you are right, it would be far easier to hide these kind of choices if the language allowed it.
However currently javascript has a purely event driven model and this means that you have trouble fitting in even a purely CPU bound computation, as it would block the responsiveness of your UI (or other concurrent requests if your are doing server side JS). That's why there was a need for creating something like WebWorkers.
