If you are an employee, take a moment and think on the following.
Picture yourself as a private contractor to your employer, you are your own business. You have overhead (personal bills), a lease (mortgage/rent), and other responsibilities in life beyond your client (your employer). Take a moment to figure out the complete cost of doing business for yourself, and then the compensation you receive from your client.
Let’s say you have a retainer agreement with your client, where they pay you X amount per month to do X amount of hours of work for them. Once in a while you might do some extra work for them without charging them for the hours over and above what they had allotted, for goodwill purposes. What would you say if your client told you he expected you to work extra hours all of the time, so now you’re doing X+10 hours for them each month? How about if you are expected to work through the weekend for that client, do you charge the same rate?
With this type of perspective on your position, how does it change the amount you seek in salary requirements? How about in overtime worked when in a salaried position? What will you say when your review comes around?
You are a private contractor. The company is paying you for your time. When they ask for extra time, it is not wrong for you to expect extra compensation for it.
This is a broad generalization, but food for thought none-the-less.