Pretty sure that this is the same idea as what I just saw in Lisbon a few weeks ago (they had red umbrellas). I talked with a couple of the young guides at the Praca do Comercio and learned that this is a way for unemployed history, art, archaeology, etc. grads to make enough money to squeak by. They are knowledgeable and peppy and hoping to make a living.
So, irrespective of what you think about "tipping in Spain", referring to the recent thread, tips are definitely expected and generous tips are very much appreciated. If this is like the Lisbon operation, these guys get no salary and live off the tips.