Jogger stroller

So I just learned yesterday that you're aren't supposed to jog while the baby is still in the carrier. This is my first baby so I'm still learning. About how long will the baby have to be in the carrier for? I was really looking forward to being able to run with the baby, that's why we got a jogger that has the infant seat attachment. And I dont quite understand, the infant carrier is a rated car seat, so if it's safe enough to be used in the car why can't I use it to run a 6 miles per hour pace?