Taken from the rules:
1.7a Prop Abuse
No abusing props (grabbing them with the magneto stick to shield yourself, push players away, kill players, to climb or fly with them etc.) [The poltergeist traitor weapon or other prop manipulating weapons void the no prop killing/pushing rule]
Let me highlight this part to you: grabbing them with the magneto stick
to shield yourself. I don't know about you, but hiding behind a prop seems suspiciously like prop shielding to me...
Im pretty sure the rule was meant to mean specifically grabbing a minecart and waving it around wildly infront of yourself to block shots, causing potential harm to others as you swing a minecart around wildly. Sounds very fun police to start punishing people for building minecart forts.