If you're coming down from orbit, then you'll be going just a bit less than orbital velocity of 17,500 mph.
Earth escape velocity is about 25,000 mph. That's what you need in order to get to the Moon or Mars. As it takes lots of fuel to get that, it's too difficult to carry enough to slow down much on the way back. Hence you brake using the atmosphere.
From memory you only pull about 3G on normal re-entry, whereas the astronauts returning from the Moon had to put up with something like 6. And Apollo 13 was more, because they got the entry angle slightly wrong.
Obviously you want to go to Mars as quickly as possible, so there's a balance between how much you accelerate to speed up the trip, how much fuel you can take to slow down - and how much pain you're willing to put up with on aerobraking. I guess this is another reason that they want to take their Earth re-entry craft with them all the way to Mars, as it woulld take too much fuel to be able to slow back down to orbital velocity and rendevous with one (the craft may be lighter than the fuel otherwise needed). Also the Orion is a lifeboat, as you can abort directly to Earth if the rest of the Mars ship breaks down. NASA presumably decided the AA were too expensive...