What's wrong with the factory owner owning the product of the factory is that then he has the power to set wages and prices, the employees do not have equal power to do so. This creates an inequality of power, which means that it is no longer an anarchist situation. The owner is a government.
The factory owner is subjected to competition from outside. Laborers are subjected to labor competition. Supply and demand determine the negoitating power of factory owners and laborers.
IF the factory owner is desperate for labor, he may then spent an exorbitant amount of money to acquire said labor. In this case, laborers seem to be in a position of power.
However, if the situation is reversed, the factory owner is now in position seem to be in a position of power.
BUT, this is what factory worker and the factory owner agreed to. Libertarian ethical theory said this mutually agreed agreement cannot be interfered with no matter how unbalanced the power relation is.
Indeed, you may have seen me out there just BEGGING for jobs. The power of employers are great.