Yes, you are supposed to tip them. Do you realize how heavy those beer/peanut carts are to carry around?
If you don't want to tip, I suggest going to the concession stand, but if you get your beer from the person walking around, please tip them. That's why they are working so hard; for your tips.
Peachy, I've been a season ticket holder for over 20 years for a major collegiate football team. I see people buy stuff all the time. And since we have aisle seats we often hand the product/money/change down the line. I have literally never seen tipping happen.
I'm not saying you're wrong or that the hawkers don't work hard and deserve it. But this might be a regional thing.
I've also been to some minor league baseball games and had the same experience.