Take for example, the solar energy. What does it cost an individual to receive the solar energy? Isn't it free, then? Oh, ya I forgot! There is an opportunity cost. If you choose to have sunbath then you give up the possibility of using it for your solar cooker. But isn't that's like saying you can't have two free lunches?
I think many times people wish to say that one can choose only one alternative (isn't that's why they are called alternatives in the first place). Whether each of the alternative is free to you is immaterial. In that case, the proper phrase to use is, "You can't eat the cake and have it too!". And the cake in question could be falling freely from heaven!