Perhaps the difference is that we know that when sending emails we are sending data out, into thin air, unknown, world wide space, and that anyone and anything can and possibly will intercept it. Just as letting a balloon go up, we lose control of what might happen to it. We do not have the same expectation for data on our computers. Those we treat as part of our "my home, my castle," where any intruder might be shot on the spot (well, only in some states though).