If you view your employer as a master who gets to tell you how and where and when to work, for the pittance he affords you so you can live to work another day, and so he can take all your extra productivity and initiative and IP and profit from it as he wishes, then I guess you are right.

If you view your contract with your employer as a mutual agreement where you provide a set amount of work for a set amount of money, then your productivity is your own. You can use it for a better life or more money.

I guess this is why I contract. I happen to think my pay should be directly linked to the value I create, rather than an arbitrary amount based on how much it costs me to live, and how long it takes me to find work.