This is actually by design as it's currently implemented. However, support for partial-hour work days is an increasingly popular enhancement request, which you can also lobby for via our Product Feedback form at http://www.daptiv.com/support/product_feedback.asp.
In the meantime, a solution that has worked for many of my clients is to simply reduce the number of hours available each week to a value that will work for planning purposes, in this case 37 or 38 hours. It's not the perfect solution, but it has been sufficient for the purposes of Capacity Planner, which is really a relatively imprecise measure of a resource's availability compared to the Actual Work a resource or group of resources is assigned. It is intended for high-level resource planning and forecasting purposes, so having each resource's availability one half hour greater or less than what is considered their real capacity should not create too much disparity between Capacity Planner and reality unless you have very large Resource Type pools.