Although we may interpret "premium" to connote "better", the usage meaning "something you have to pay" is accurate.
Perhaps we could take back the language and start referring to the extensions as "Media Fee Extensions"?
I think in this case it's just a meaningless word introduced to make it sound better than the "proprietary plugins" like Silverlight and Flash. I think their alternatives were naming it like "Awesome Viewing Experience Extension" or "Netflix True HD Vision Extension", or something like that.
In each case they wouldn't mean what they normally mean, and would be used in a misleading way just to give you a "good feeling" about having DRM on the web.
But the accurate words have negative connotations so they avoid them on purpose and try to use positive ones which may be technically accurate but wouldn't be how anyone would normally choose to describe them. This is the definition of spin.