[jurre]

Parsing the referrer would still require some work from their servers right?

[hifier]

Sure. I suppose that could be the reason. However, something like HA proxy[1] and (IIRC) CDNs like Akamai can be configured to inspect the headers and take various action.

[1] http://cbonte.github.io/haproxy-dconv/configuration-1.5.html...