I'm not sure where everyone is getting that you would allow your child to ride in the front seat. Was there a post that was removed? I'm confused...
Anyway, some babies go through a phase where they just do not like to ride in the car seat for whatever reason. As long as your baby is safe, secure in his/her car seat, isn't overly hungry or needs a new diaper, and isn't too hot or too cold, try to tune out the crying.
When my son was about 6mo, he literally SCREAMED for the entire 67 minutes it took us to get from our hotel to my friend's house (we were visiting out of state), and it was probably the most nerve-wracking 67 minutes of my life to that point. But what CAN you do? He was safe and secure, not hungry, etc. He just didn't want to be in the car seat.
Both my kids went through this phase from about 3mo til about 5mo, and we haven't had a consistent problem since. I think part of it is the frustration of being confined to such a small space when they're at a very important developmental stage (creeping, crawling, playing, etc).