It's because of the way the DMCA is written; there's a vast scope for actions that are by default illegal and need 'exceptions'.
The LOC defining those exceptions instead of some agency setting administrative rules (which is normal procedure for things like this that are too specific and move too fast for congress) is just an artifact of the US gov't not having a clear agency to deal with tech issues like this.
The LOC defining those exceptions instead of some agency setting administrative rules (which is normal procedure for things like this that are too specific and move too fast for congress) is just an artifact of the US gov't not having a clear agency to deal with tech issues like this.