Yes, it is fair that giants are never measured. There is no reason that a giant needs to be measured. People can see that a giant is a giant. It would be interesting, however, to know how large the giant is. This circus rule has a good side and a bad side.
Part of the appeal of the circus was the myth and the magic behind it. By not allowing themselves to be measured they helped to add to that mythology. It meant that Giants could claim to be larger than they really were and use tricks of perception to help create and illusion without fear of being discovered.
In the circus, it's not about how something really is, it's about how something looks. Performing in the circus is about making things that look spectacular. There's no reason that they have to give away their secrets just like a magician shouldn't have to give away their secrets. It's their choice as a performer.
Some of the Shortest Man in the World guys had some hilarious arguments, especially when they were pictured standing next to each other. How would you know you had the world's tallest man when you don't know how tall the competition is? This is probably why we don't have real giants anymore. Once you stop measuring them it is hard to pair them up effectively (as the most famous circus giants with naturally marry each other) and nowadays our tallest people (NBA players) make too much money allowing them to procreate with people of all heights.