These are just conventions, one can pretty much do whatever they want in their applications. At the same time convention has its own advantages (most of the times, think about code maintenance). Its still along the excepted line as long as the mutations are side effects of the GET request. Somewhere down the line the intent is to provide a separation to easily understand the systems.

The GET request is issued to the web server for content of the HTML page and then all the scripts. Then the scripts issue POST/PUT requests to the analytics server updating the count.