Since when are fees required? Well not required, but since when is it customary to send. I thought transactions were free under two conditions (size limit and a minimum amount; they're somewhere on the bitcoin.it wiki). You earn $3500 for a single block, why do we need to charge fees now?
And I also wonder how miners set this. Miner clients (or pool software) just has a configurable setting for the minimum fee or so?
A fee is not required, However it is highly recommended if you want your transaction to be dealt with soon. For example, if you ran a business and some people donated and some didn't. Who would you help first? Maybe you would help the person who went first, but Bitcoin has many transactions a day. Putting a small fee increases the likely hood of it getting 'dealt' with sooner rather than later.