Tells you what the real aims of the course are
Ecmascript (aka javascript) is an important language in that is the most widely supported language for running code in a browser. If that is what you want to do then its the right thing to learn. I really don't see the point in running it elsewhere, though I will concede that there are some advantages to using the same language in both client and server I personally don't find the argument for its use on the server side compelling.
Though I'm hugely impressed by the performance of modern javascript engines and the utility of libraries like jQuery I actually don't like the underlying language much.
I recently went through the process of porting my 20+ year old classic fractal hack
https://spbooth.github.io/xmountains
to javascript. Once I got past the superficial resemblance to Java the language actually reminds me more of perl. Very useful in the appropriate context. Large number of powerful libraries available. But you spend far too long debugging small problems that would have been flagged as compiler errors in most languages.
Not knowing any javascript can be limiting, only knowing javascript more so but heck everyone has to start somewhere and at least if you teach javascript first you don't have to worry about students installing the development environment and also gives immediate access to pretty graphical results. Kids these days just don't find hello world impressive any more.