There are a couple of ways of looking at this: One is the very secular nature of Spanish (and European) society these days, especially young people. Many are not raised in church-going families, and they do not know how to behave inside churches. Worship and respect for holiness are just not part of their world. They aren´t so much disrespectful -- just ignorant.
Historically speaking, the church was for centuries a center for socializing. It was pretty and cool inside, there were places to sit down and private corners where you could make deals and flirt and hide things. Dogs and cats wandered in and out, and vendors sold their candles and fruit and god-knows-what in the nave. Business went on whether or not a Mass was being said in one of the chapels or a body was being interred under the floorboards. (Images of these things abound, esp. in Dutch art from the 17th and 18th centuries.)
Even now, in Christian churches all over Eastern Europe and the Middle East, people wander around the church during services, lighting candles and (quietly) visiting one another while the Mass-goers just ignore them and bells clang overhead and babies cry. It´s just another part of living a day-to-day noisy Christian life. Worship is not a quiet time, set aside for quiet moments, because there aren´t very many of those.
Still, I don´t think it is too much to ask of the tourists (and rackety pilgrims) at Santiago Cathedral to chill out and be quiet for an hour on the bench and let others worship undisturbed. I´m glad they´re enforcing the Mass rules now.
Reb.