Having a platform that is already out there and which you can execute against without even an install might not prove much more about the merits of a language, but it is a merit in its own right.
Of course, in this particular case... JavaScript isn't exactly widely deployed in hardware, and this device is what provides the platform so... yeah.
> It's widely adopted because Javascript has a stranglehold on the browsers and people don't have a choice.
That's not entirely true. People have had lots of choices. There was Java. There was Flash. There was even VBScript on IE (I bet you didn't know that), not to mention all the EMScripten fun that is now available. Time and again, people choose JavaScript if for no other reason than for its lowest common denominator qualities.
> Nowhere in the software space would such a monopolistic position be acceptable, but hey on the web for some reason it's OK.
If you consider the list of widely used languages for which there is an approved standard by an independent standards body and multiple independent implementations, you end up with a surprisingly short list (and some qualify but by the skin of their teeth). I think, sadly, this is actually quite widely accepted.