The principal intention was all of the above and also a response to the popular demand for ballads attributed to folk tradition. Yes, once the upper classes did all they could to erradicate true folk culture, especially rowdy festivals and all the traditional art forms that went with them, the same people were in Percy's time searching for artifacts. Where they could not find them, they manufactured them. Percy is not a credible source:

Thomas Percy 

And no, tampering is not a needlessly tendentious word. It's the word that fits.

When a people are conquered, it doesn't have to be by a foreign entity. It can be by elements within one's own culture. But once the conquering is done, the winners always want their artifacts and their museum. They want to be tourists.

In the period covering the Enlightenment through the Industrial Revolution, if you were poor, your very essence was under seige. Everything "primitive," "ignorant," and so forth became the object of contempt, its value was limited solely to how it could be used as fodder for "progress."

That's why people should be skeptical of claims of this or that originating from the folk. The people who write history tend to cover up their wrong doing and the sufferring that resulted from it. And all of this artifact business puts a positive spin on a grim situation.

That is not to say that there were no honest folklore collectors. John Francis Campbell for example, learned Gaelic and faithfully recorded the stories and songs of the people of Islay. But there are also those who faked it for personal gain. Percy's one of them.