As you can guess, this approach can lead to many people perceiving scope changes and customer collaboration as a nuisance at best. However, agile project management recognizes that we know the least about a project and uncertainty is the highest at its inception stage. Making decisions in bite-size chunks with constant customer involvement as you learn more about the product makes more sense than making one, all-encompassing scope decision at the beginning of the project.
Having said that, flexibility to this degree is only possible with constant customer collaboration. Understanding this allows both the customer and the development team to work together side-by-side as one team towards the same goal. After all, both want the project to succeed.