I agree completely. I think this is simply some technical debt incurred by the way in which TeamPulse has grown over time. Back when those screens were created the Backlog Priority concept did not yet exist in TeamPulse, and it would seem that when we did implement backlog priority into the application we simply neglected to update the decomposition screen to make proper use of the new functionality.
I have created a new Feature Request on your behalf, you can view and vote on it at the following URL: http://feedback.telerik.com/Project/8/Feedback/Details/660
All the best,
the Telerik team