I think the important thing here is to look at this as a general tendency rather than a hard and fast rule and the barriers as brick walls. Take my place as a for-instance. Generally, lone gobblers won't cross the bigger creeks. The do, but generally they won't. Once in a while they will, but mostly they'll honor my calls from the far side and then go the other way.
I used to frequent my one bottom quite a bit in the early season, and it was funny. Most times, I would not even need to call. There would be hens on my side, gobs on the other side. The hens would call like crazy from the roost. The gobblers would answer, but they never seemed to meet up-- come flydown, they'd stick to their side of the creek and each would pitch down and go up the hill away from their friends in the bottom. That's mostly what I saw. On the other hand, I also frequently saw mixed flocks pitch down from up one of the little hollows and come right down to the creek and cross it and go up the other side. Now why I never saw hens from my side meet up with gobblers on the other is a mystery. So is why a mixed flock seemed to cross creeks like they weren't there.
Fences? They'd starve if they didn't move through fences on my place. However, I see a tendency for them to travel along fences and even the old fence rows without fences, before they will actually cross them. My guess is that they are trying to check things out on the other side before they cross.
They also seem to have set crossing points-- a lot like deer. When I'm in the fields day after day, they often go through the fences at certain spots, and these spots remain stable generation after generation. If you look at these spots, there's nothing particularly different about them-- this 10 foot stretch vs any other. Still, it must mean something important to the turkeys.
I also think there's some kind of . . . well, for want of a better term, rudimentary etiquette going on with the turkeys. If a gobbler comes up to a fenceline, I think he expects he's going to have the hen meet him halfway on it. She'll at very least show herself. He stands by. He waits. He struts. He gobbles. If she doesn't show up, he thinks she's not interested. Before he commits to flopping over, he at least wants to see SOME positive intent from his partner.
Now all of this is not written on stone tablets, and a lot of what looks like no barrier at all can be perceived to be one by a gobbler. For instance. I've seen a gobbler hang at the edge of a shadow in the field. The reason? I don't think it's an impenetrable barrier to a gobbler, but I think he's trying to discern what's in the shadows. He doesn't have a hat brim. He can't throw a hand up to his forehead and block the light. He has to move back and forth until the situation in the deep shadow finally resolves into an image he can be satisfied with.