Ummm ... is this not how all software works? Is the average person 'aware' of the code that is executing as he/she clicks around in any piece of software? How is a web browser any different?
It happens all the time but for some reason, it's more evil when it's javascript.
JS is much more foisted upon users than MS Word ever was.
No signatures are checked; for none of those sites is there a validation whether it came from a trusted publisher and a prompt to install it.
With regular software, you explicitly download and install and in newer operating systems, there are certificates and signatures to give some modicum of protection. If you install something unsigned, you give your explicit consent.