It's not a rule that's needed, like tvbcof said, a line needs to be drawn.

Building price discovery into the Bitcoin P2P network will allow the Bitcoin relay network to thrive under the successful economic policy of free markets. It’s the answer to the question, “How can network parameters, such as the largest acceptable block size, be set in a decentralized way without resorting to hard-coded mandates?” Pricing of network services would allow participants in the Bitcoin P2P network to efficiently manage the scarce resources that the network needs to function.
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Put another way, with the current design of the Bitcoin P2P network, relay node operators bear the burden of bandwidth costs produced by other users. This is because there exists neither a method of paying for network services, nor a method for discovering the correct price of network services. The lack of a pricing mechanism for these services effectively pegs their price to zero. This incentivizes other participants in the Bitcoin economy to treat the services like they’re unlimited instead of scarce and over-consume. As in the case with other top-down networks, the only available response is to introduce new mandates to control consumption.
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Attackers with sufficient resources who do not care about profitability can break Bitcoin via a wide variety of vectors. The only possible defense against such attackers is for Bitcoin to grow larger than them, as rapidly as possible.