Not only vent air from the house to allow the drains to work correctly, they also allow septic field and tank gases to vent to the top of the roof vents. This is normal, and indicates that your septic system is probably working properly.
The end of your septic drain field most likely has a vent(s) to allow air into the system. this air passes through the drain field pipes, over the sludge in the tank, and up to the top of the roof.
You should pump your tank every 2 to 3 years maximum to make sure you don't have excessive sludge buildup.
I know of no way to eliminate the smell, but I have heard of people covering the inlet vents at the end of the drain field on a temporary basis to minimize the smell while having company over, etc.