In fact apps can register for voice commands since the release of WP8. Microsoft just improved it so you don't have to register e.g. 1000 movie names to enable the WP voice recognition to understand them, but telling WP to listen for a movie name.
But I thought Google Now doesn't offer an API either? I use both Siri and Google Now (I have an iPhone and Nexus I find myself regularly switching between) and I feel like they are two different products, not meant to compete (yet). Yes, Google now can answer questions like Siri but because it's main focus is on showing me stuff I need when I need it I tend to forget about the Q&A part of the product. I also feel that's Siri's Q&A is better. Google Now needs to pull in more sources and open an API to keep competitive with Siri. Siri needs an API and more contextual awareness to keep competitive with Google Now.
