Why Don't Companies Release Specs?

Mhrmnhrm asks: "With the recent activism by the OpenBSD crew focusing on release of documentation from the likes of Adaptec, Intel, and others, I'm left to wonder: why do companies insist on believing that by denying access to the specs, they somehow gain an advantage? It's not like telling a programmer how to communicate with the underlying hardware is going to tell them how it (the PCB/silicon) was designed, so why make this information secret?"