Uh Miners? You make it sound like there are testnet pools and such. If you are lucky and catch the right moment you can solomine a block with your cpu. So while I theory you are right, it makes little sense to charge a fee for the testnet.
I thought that Testnet was created to test Bitcoin features without risking any money. If fees are part of Bitcoin, shouldn't they be a part of Testnet too?
There are no fees mandated by the protocol only by clients and the rules imposed by a client depend on the client version. testnet nodes tend to not be updated as quickly. Still using testnet in a box you could construct a multi-node test environment to test the processing of min fee to relay rules.