In my opinion, copy protection costs a fair amount and people pirate games anyway. You can use Starforce, Tages, Securom etc. but all of these can, and will be bypassed in a matter of hours or days - even before the official release date, if The Scene can get his hands on an early copy.
Amanita made the right choice, they don't sell their game for too much (like some indie groups who are now bust or close to it because of being stupid) so people who value their work and are able to afford it, will buy this masterpiece. If they don't then they would've pirated it in any case, regardless of a copy protection.
Besides, people who
rent the game from Steam will have DRM on their copy anyway (at least I bloody hope so)