You would need to look at the history to see what happened. I would trust the scenario designer - it's a hallmark of this game that a lot of research goes into producing historically accurate scenarios with as accurate as possible troop displacements. I would guess here the designer is trying to simulate what actually happens (and happened) if a force is trying to bridge a crossing - they would get some holding foot troops across the other side, to secure the far bank whilst construction took place. There are various methods of getting them across that don't rely on bridges, obviously. Swimming for example. But it's a holding measure until the remainder plus the motorised units can use the bridge the engineers build. The Axis built crossings at these points historically, and I guess they would have had to use these techniques to do that. In reality, I very much doubt the Our river is really completely uncrossable to infantry (as it is on this map), even in Winter flood. But putting across a small holding force by some slow, dangerous, costly method is different to planning an offensive to cross that way. That's not what happens, and not what happened historically. So the scenario designer is trying to recreate that fact for you. Those foot troops were put across before the scenario started, to pave the way.