Mars gravity is a lot less than that of the Earth. So, the force that is pulling the probe down (the weight of the probe) is smaller and the drag that is required to slow down the probe to terminal velocity is also smaller. It might take a longer time for the parachute to slow the same mass to the terminal velocity though. When the probe has fallen to a certain height, the retro-rocket takes over. This method has been proved to work by Russia and murrika.

Does the "7 minutes of terror" refer to the time the probe takes to go through the Martian atmosphere until the opening of the parachute? I wonder how long a similar reentry on earth takes-- for comparison.