In that case, the site providing DRMed video and the NaCl player would have distribute an H.264 decoder (and potentially other encumbered stuff) to end users as

In that case, the site providing DRMed video and the NaCl player would have distribute an H.264 decoder (and potentially other encumbered stuff) to end users as part of a NaCl executable. It's much more convenient for a company like Netflix not to have to distribute the DRM component when companies like Microsoft and Google are willing to do it instead. Also, NaCl doesn't integrate with the HTML media elements, you'd need to implement the whole media stack in NaCl instead of just the DRM part. Furthermore, since NaCl is sandboxed by the browser (and the browser is untrusted by Hollywood in the DRM trust model), it can't conspire with the GPU on low enough a level to hide pixels from the browser and the operating system.