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.
You need to read more about how bitcoin works. First of all, the bar doesn't send blocks anywhere since it doesn't mine. It doesn't even send transactions, it just listens for incoming transactions and incoming blocks. Secondly, transactions are signed before they're broadcast to the network. Once they have been signed, they can't be tampered with. So your MITM can't edit any transactions. What he can do is be selective about which transactions he forwards where. If he were to collude with the customers, he could help facilitate double-spending attacks against the bar, but as mentioned previously any such attack could quickly be detected.