While they should be protesting the pipeline that supposed to be coming through that area it is possible that people are there to experience other things. However, I think the protest of the pipeline should be more important. They should be focusing on the one thing rather than the other stuff.
Some people went to the Occupy movement for the same reason. They really didn't care what they were protesting. Rather, they just wanted to be angry about something. They wanted to hang out with other people who were against the government, too. This is the same motivation for many protesting at Standing Rock.
The Standing Rock protesters are trying to understand and empathize with the loss of tribal land over generations by the various tribes to the U.S. Government. The protesters are learning the art of civil disobedience in the face of institutional advances. The protests have been largely peaceful and just getting to North Dakota to protest is not an easy task, which downplays any argument that the protesters are just there for fun.
Yes, Standing Rock protesters are probably right to complain about people coming for a "cultural experience." There are many people who choose to be involved in the latest protest or newsworthy event simply to say they were there. Also, the fact that Native Americans are involved makes it attractive to individuals who just want an experience.