I'm with eyeball on this one. It's simple mechanisms of suspension of disbelief. The creators of the show have already put us in a world where zombies are real and have used the storyline to attempt to explain some of the how, though a majority of that remains a mystery intentionally as part of the carrot on a stick for the viewer. That said, they have not introduced us to any reason why basic physics is any different in this world. That is why the van scene as well as a lot of the absurd gun shootout scenes bother me. Hey... ammo is heavy, they have an infinite supply, they never reload, there is never any recoil. Headshots in the dark!
Don't get me wrong, I do love the show so whatever on the physics stuff. I do have a hard time ignoring as it's happening though. Suspension of disbelief in fiction is something every writer needs to be mindful of. For me, some of those details are the difference between a good and a great story. Taking place in a world where zombies are real.