The same way OKW gets reinforced barbwire, USF gets unique "sandbag" which not only provide cover but also can potentually deny enemy armor movement.
Also its questinable really if tank traps are better version of sandbags. While it can sustain more punishment and it can deny armor, it also makes your squad model clumped up and be more vunerable to enemy nades. You can always build two of them near each other to remove this disadvantage, but in this case enemy will be able to use them as cover aswell even if they are built very close to the flag. Aswell as unlike sandbags, you can hear them being built in FoW.
Yes one has to build two of them but they have end up being a more durable version of green cover and imo that is not what they were designed for.
So yeah, in other words, I think USF having tank traps is more of a deliberate chose and unique mechanic, rather then un-untentional feature.
The issue is not relevant to USF having tank traps or not. Giving them tank traps was deliberate choice and probably had to do that with the fact they had worse vehicle mines and not because USF needed a superior green cover.