Scales are a different case too. You are only weightless for a brief period. When you jump you impose an additional force upon the scales before you leave them. When you land there is a force additional to your weight momentarily imposed also.
If you jump up and down in an airplane it will momentarily enjoy the benefit of the absence of your weight, then when you land (and jump) each time, it will pay a penalty in additional "weight" for your jumping. The net effect will be zero.