There is no way that every transaction signer whose transaction is added to a block should be considered a client of the miner of that block.
Most likely none of the transaction signers qualify as clients of the miner. Transactions are transmitted over the network anonymously via a gossip protocol, and hundreds or thousands of miners have the chance to include (or not include) any transaction in a block. Transactions are selected for inclusion in a block effectively randomly, through an entirely mechanical process, and no relationship is established or maintained between the miner and any transaction sender.
In order to assert that a transaction signer is a client of the miner that builds the block that includes the transaction, you would have to redefine what the word “client” means.
> There is no way that every transaction signer whose transaction is added to a block should be considered a client of the miner of that block.
This does not seem obvious to me; even if "client" is too strong a word, the transaction signer and miner have some social contract that's very similar to more traditional fiduciary duty, even if the technical details and enforcement mechanisms are totally different.
It is so curious that you use the term “fiduciary duty” to describe the relationship between a miner and the signers of the transactions it includes in its mined blocks.
The primary rationale of fiduciary duty is trust. In contrast, the whole reason that miners even exist is so that the service they provide can be performed in an entirely antagonistic environment, without trust.
I suppose it can be said that a miner’s “social contract is very similar to more traditional fiduciary duty”, but only in the sense that a thing is somehow conceptually related to the exact opposite of that thing.
Many cases of front running involved 3rd parties with access to client information. Anyway, my point was legally the information may be protected even if the protocol is open, though I seriously doubt that it would be considered so stranger results have happened.
Index front running is not illegal and is based on public information. So, this may be a form of legal front running.