It's mining and relay node _policy_, not a protocol rule. Please don't confuse matters by using vague language when you can instead describe exactly what it does. This works exactly the same as the other "standard transaction" rules— zero value outputs, multisig with a dozen keys, outputs less than <0.01 without a fee (initially of 0.01 later reduced to 0.0005, and to be reduced to 0.0001 in 0.8.2), etc. Transactions 'breaking' the IsStandard rule do get mined.
Consider the user experience if a pure miner change is implemented:
You send a transaction. It reliably gets to the 10% of miners still implementing the rules you depend on, and it takes quite awhile to get mined. Annoying, but your transaction gets mined, and the person you sent it too sees it immediately.
On the other hand, with a relay-related rule, unless there happens to be a path from you to that miner of relay nodes willing to relay your transaction, absolutely nothing happens and you are left scratching your head wondering why.
Like it or not, changing relay rules is a much bigger change than chaning block inclusion rules, and it's much more effective at blocking transactions until a high number of nodes change their settings.
Sorry, but calling it a "network protocol rule" sounds pretty accurate to me. It's not a blockchain rule, it's not a block inclusion rule, it's something that decides if a transaction can propagate on the P2P network.
Having read jdillon's stuff, I'm pretty sympathetic to the idea that we should be accurate in what this is, blocking. Yeah, it leads to massive forum threads, but again, I agree with him that the fact we're seeing that just shows how little people understand where Bitcoin is going.