They're getting their message across before bargaining begins in earnest. Showing a lot of visibility and political clout means bargaining will be influenced by not just what happens at the table but what happens in the community.
If you cut wages by two-thirds, that creates a class of people that can't afford to go to Wal-Marts. If other industries do this, America will have a far smaller middle class, and the whole economy in the 21st Century begins to unravel.