Allow me to address each of your assertions individually:
The employer hasn't added any work to the widget. It's the same as when he got it from the employee. The employer gets 720 bitcoins without having done any work.
No, it isn't. It's packaged, assembled into a group with other like widgets and
then packaged, or compiled into a full device by adding other doodads and whatsits and widgets, and
then packaged. It's changed in a myriad of ways, and each of those processes he had to either do himself, buy the machinery to do, or hire someone else to do. There is considerable further processing to be done after the initial production. Don't forget other costs, such as overhead on the factory itself, maintenance on the machines, marketing expenses, etc.
The employee concedes those 720 bitcoins and submits to his employer's direction because the employer will prevent him from working otherwise.
I think, perhaps, you ascribe too much power to the average Capitalist. The only people who have enough influence in a field to blacklist someone are the "captains of industry", the Bill Gates and Steve Jobs, as it were. Nobody is going to care if you quit say, Trend Micro because they weren't paying you enough, Especially when you offer your talents to their
competitor. Additionally, let me remind you that there are always those who do not listen to the "power elite", those willing to bypass Bill Gates saying not to hire him, if his skills are good enough.
Even if his former employer doesn't blacklist him, the worker will encounter the same deal with every other employer.
Not so. In the free market, there are options for every level of compensation. If his skills are not sufficient to warrant a higher rate of pay, then he is free to seek employment using a different set of skills, or apply those same skills on his own, fashioning widgets from his own raw materials, packaging and marketing them himself, and if his widgets are of a competitive quality with those of the company he left, he can charge a comparable price, gaining the profit for himself. Of Course, if he starts selling too many widgets, he may find that he can't keep up with production and have to hire an employee...