Those legalities have a basis in whether or not what you are doing would infringe on other people's rights. In the case of building a nuclear waste site in your backyard, your neighbors object because it poses a grave danger to their life that can only be regulated through your competence, and they most likely are not willing to place faith in your competence.

Playing your DVD that you bought in whatever manner you see fit, on the other hand, harms no one except the people who would like to charge you a toll for using the product you already paid for. In any case, it does not infringe on their rights at all, only causes a loss of potential revenue. That is why DMCA is stupid; it is not protecting rights of copyright holder, only revenue stream.