So if nobody can steal anything, what's the problem?
What do you mean? The MITM spoofing the node gets to run away with all of the BTC that the pub took that evening.
How? It can't get the bar's private keys, and it can't change the bar's receiving addresses. So how is it supposed to get hold of those coins?
The bar thinks the man with the suitcase in the corner is actually a node. The bar is relaying blocks to the rest of the network through this supposed node. The blocks either get edited or just not relayed to the rest of the network. The man with the suitcase pretends to the bar that the block has been confirmed correctly. The man with the suitcase walks out with BTC.