The general assumption was that if Russia ever had attacked then Germany would have been the battlefield. Nobody in Germany expected the country to survive in that case. It sort of would have made sense to try to stop the advance at any cost.
One problem with nuclear mines is that they're unattended. If some crazy circumstance happens where the US loses control over the area, but it's not war, then you suddenly have a bunch of hidden nukes lying around that could detonate.
Another problem is that if the Soviets ever found them then they could use them as effective propaganda. If even one of those mines goes off, then they could say that they're liberating the Germans from their American oppressors. After all, you don't nuke your own ally and a nuclear detonation is hard to hide.
