This type of tracking seems to assume the user is not bothering to send a fake Referer, e.g. she can just use the URL she is requesting, or just omit the header

This type of tracking seems to assume the user is not bothering to send a fake Referer, e.g. she can just use the URL she is requesting, or just omit the header. One could argue such users are "low-hanging fruit". Very few websites will vary the response if there is no Referer. Sending it really offers little benefit to the user. Setting up a "headless" browser also seems like overkill. Firefox 57 and later has a -screenshot command line option which saves a PNG. No need to launch X11 for this to work.