I don't understand your point. Open Source is a meanless term because it means potentially lots of different things depending on license type.
Castle would release their code under a GPL license to address the issues raised by Peter Wild and to try to create a much bigger target market of which they could take a slice with support and commercial licensing. I cannot see Castle releasing it under a BSD or Apache style license.
They do this because it makes better business sense than keeping the code closed - no other reason.