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How I feel this.
Every transaction, 1 address -> 1 address, take a 0.0000 0001 BTC.
Attacker try to flood network with transactions. Presuppose, he have 1 BTC. Flood could looks like 1M of addresses. 1 000 000 * Fee( 0.0000 0001 ) = 0.01 BTC. Attacker could make 100 attacks with 1M addresses, thereafter he out of money.
Will network of bitcoin overloaded ? What a math behind current Fee?
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Transactions are very big. So one is very expensive.
Less transactions is better for the network.
Large transactions, with old coins, do not need to pay fee. They can not be spammed with.
Any miner can choose a minimum fee. The miner chooses the math.
The commonly accepted fee is just chosen by bitcoin developers. It was their choice, no special math involved. We try to have a manageable amount of transactions per block.
