Because the point of DMA is to give fast access to RAM without involving the CPU. That is what makes the speed of Firewire or PCMCIA or Thunderbolt or PCIE feasible. Without DMA, these technologies would not work.
Also, the password needs to be stored in memory and accessible by DMA, since you would not be able to use it without having it readily available.
Also, the password needs to be stored in memory and accessible by DMA, since you would not be able to use it without having it readily available.