Simple logic. DRM requires secret (from the user) code to run on user's machine. DRM doesn't trust the user (user is treated as potential criminal) for the sake of content owner interests. Such kind of predisposition makes it very reasonable for the user not to trust the content owner in return and to treat DRM by default as a privacy breaching malware and security risk (until proven otherwise - which isn't possible, since it's a black box). Trust is always mutual. How else can you view this?

because a drm plugin is a plugin,like flash or java applet. You'll have to download plugin DRM A,B or C to make it work.The only difference is that it will use the video tag.