Its true that miners don't set the transaction fee.But it is also true that we have earlier heard that some miners sending bitcoin with very low transaction fees that the network getting congested.But it will be only up to a limited level.
It does not matter how many low fee transactions they send. The higher fee ones will get processed with priority and traders paying the high fees. I don't think it's any "spam attack" just network filled, people pay high fee, miners process those in order of fee paid.
