It seems to be a deliberate security of Windows 10 Store apps that assemblies not be allowed to be loaded at runtime. I think that this feature is massive overkill and stops UWP development dead in its tracks. However, if it is a rock solid design decision by Microsoft, there is nothing we can do to argue with it.
Instead, I'd like to ask the question, assuming that you were not deploying your UWP app to the store, would it still be impossible to load an assembly dynamically? If it's possible, how?
Please see this feature request: https://wpdev.uservoice.com/forums/110705-universal-windows-platform/suggestions/18145291-dynamically-load-assembly