This is to prevent you moving content belonging to another profile that has been password protected.
As you mention, this does not provide much security if the profiles are not so protected, because you can just switch between them: but having the XBox behave differently depending on whether the password is set or not adds complexity, which costs money and risks adding bugs. Remember too that some of content has DRM attached to it, adding to the complexity and requirements.
And it's not necessarily about security. Won't most XBox users only have a single profile? Won't most people moving content in multiple profiles be replacing the hardware in a shared XBox, and so can use Microsoft´s migration tool? So won't most users never encounter the scenario you have?
As an advanced user, of course, you have perfectly good reasons for having multiple profiles, and even better ones for not using Microsoft's tool, but it's hard for Microsoft engineers to justify features that only advanced users need in such a low margin product.