Yeah .. umm .. and there is no guarantee that your CPU isn't broadcasting every single operation to a top-secret government satellite, either. The technology is there: have you audited your CPU today?
Build it yourself and verify via hash that what is on the firmware is what would get written.
However, if you're so paranoid that you think this could be a problem, you're already outside the realm of buying preassembled computers anyways. Who knows who could have tampered with it en route?