The code forms the entirety of the social contract. "Code" referring to officially sanctioned code that comes out of the official Bitcoin developer team. This code defines a set of rules, which enables trustless transfer of value. While I do agree with the goal (and indeed the necessity) of decentralization, I think that it can be taken too far. Everybody needs to play by the same set of rules. I don't think that some miners choosing to mine X transactions, while other miners reject them, is a good idea. There needs to be consistency between miners in order for the network to perform in a predictable manner, and performing in a predictable manner is the only way that Bitcoin will see greater adoption. (I seriously wanted to punch somebody when I sent like $500 worth of BTC in a straightforward transaction, and it wasn't confirmed for hour after hour due to the QT client not including a mining fee.... having that money out in unconfirmed limbo, when I could have included a 10¢ fee that would have gotten it included in the next block, was infuriating. But no, the fee wasn't even an option in the QT client. This is the kind of thing that makes end-users abandon a platform forever.)
The code is the Constitution, if you will. It defines the rules. There is no judicial branch to interpret the code. Just the code. There is no dicta, no Talmud, no human intervention, no wiggle room. Either a transaction fits the rules of the code, or it doesn't. Pure algorithmic consensus. This is the promise of Bitcoin, the great problem solved by Satoshi.
No, the code is just an implementation. At most, it defines the protocol. That's just part of Bitcoin - and not the part responsible for the spam filtering. Miners are another part, responsible for that.
The code of the reference client (more specifically, bitcoind) IS Bitcoin. It is the DNA that directs the cell. It is the blueprint that defines the house. You could lose everything else -- cgminer, pool management software, whatever -- and Bitcoin could survive on just bitcoind alone.
The reference client implements a fee system. If this fee system is not honored by miners, how is the average user going to be able to trust Bitcoin?
The reference policies are merely examples, and have absolutely nothing to do with the Bitcoin protocol.
Miners should be running with their own customisations of it.
Bitcoin is trustless regardless of the individual policies of any given relay node or miner.
The Qt client allows you to choose not to include a fee, if you don't want to and it thinks that you should. But, AFAIK, it does NOT allow you to arbitrarily add a fee to a transaction that it thinks should be fee-free.
You have that backward. It does not allow you to send without a fee if it thinks it's necessary, but it does allow you to add a fee when it thinks it is not necessary.