If you look at an NTSC video signal on a scope, the video portion of the signal uses the top 75% amplitude of the signal and the sync portion takes up the bottom 25%. Within the 75% video portion of the signal, the higher the amplitude the brighter that part of the picture will be. In your TV the sync separator removes the sync portion of the signal from the video portion and sends it to the vertical and horizontal circuits to synchronize the video part of the signal to the scanning of the electron gun in the picture tube. Most VCR's use this 75%/25% ratio to set brightness level, they will automatically set the video part of the signal to be 75% of the entire signal.
Macrovision varies the amplitude of the sync part of the signal from about 15% to 80%. The VCR sets the video part of the signal to be 3 times the sync amplitude in order to preserve the 25%/75% ratio. When the amplitude of the video part of the signal is less than 75% the picture is too dark, if it is over 75% the picture is too bright. TVs are not affected.