Quoting Torbutton Design Document:

It implements a form of "smart" referer spoofing using http-on-modify-request to modify the Referer header. The code sends the default browser referer header only if the destination domain is a suffix of the source, or if the source is a suffix of the destination. Otherwise, it sends no referer. This strange suffix logic is used as a heuristic: some rare sites on the web block requests without proper referer headers, and this logic is an attempt to cater to them. Unfortunately, it may not be enough. For example, google.fr will not send a referer to google.com using this logic. Hence, it is off by default.

Tails has kept that default for the best web browsing experience.