Why not? The workers have created enormous value, more than any other time in the history of humanity. Why shouldn't we frame this in terms of where that value goes to?
People have rent to pay. The "you agreed to it" argument only works if the person genuinely had other realistic options available to them. Nobody would work for peanuts and pee in bottles if they had another realistic option.
It's not about his wealth. If Jeff had been independently wealthy before starting Amazon this wouldn't be an issue. The problem is the percentage workers receive of the value they generate. Jeff has gotten rich by pushing that number as low as possible, while also working to kill off businesses that might offer alternative employment to low skill workers.