This is the fact that a low wage constitutes a competitive advantage for less-skilled workers that serves to protect them from competition from more-skilled workers. In other words, a wage of $7.25 per hour for fast-food workers serves to protect those workers from competition from workers able to earn $8 to $15 per hour in other lines of work. The workers able to earn these higher wage rates are not interested in seeking employment at the lower wage rates of the fast-food workers.
Between less employment overall in the least-skilled lines of work such as fast food, and the incentive created for vastly increased competition for employment in those lines coming from more qualified workers, the effect could well be to close those lines altogether to the employment of workers at the low end of skill and ability. That, of course, would deprive these people of the opportunity to acquire skills and abilities from work experience that otherwise would have enabled them to become capable of performing more demanding jobs later on.
No one can question the desirability of being able to earn $15 an hour rather than $7.25 an hour. Still more desirable would be the ability to earn $50 an hour instead of $15 an hour. However, it is necessary to know considerably more than this about economics before attempting to enact sweeping changes in economic policy, changes to be achieved by attempting to organize a mass movement that is based on nothing but a desire for economic improvement and no real knowledge whatever of how actually to achieve it.