Thanks again for clarifying. Technical aspects of the network have never been my strong suit. Just another daft question: If nodes do not police the chain, then why is it advised for nodes to have a full copy of the blockchain? I always imagined it was for situations like the one discussed, to prevent malicious miners from propagating invalid chains.
There is a difference between the validity and the honesty. The onus of maintaining a chain that is mined with fairness and integrity in mind is on the miners. No one on the network can judge which is the honest chain and which one isn't, and the only assumption that we can make is to assume that the majority of the network are honest and not engage in double spending, malicious chain-reorg, etc. Keep in mind that executing a 51% attack doesn't break the protocol rules.
Nodes play an important role in being economic agents in ensuring that the blockchain that they see is valid and for SPV nodes to be receiving the a chain that is valid, keeping in mind that SPV nodes has no idea whether a chain is valid or not. Nodes are useful for individuals to ensure that the chain that they are seeing is valid.