It may sound weird to you but if the owner of a house does not bother to install a lock or get a key made and someone walks into his home we have a perfect case of trespassing. Trespassing is a crime and will be punished (again, don't believe me, ask your lawyer).
Maybe an unfair question: Do you personally believe that YOU are entitled to walk into someone elses home if their is no lock to stop you? And, to take things a little bit further. Since you are there you might as well take the stereo, the tv, the pc and whatever else you want with you. Do YOU personally believe you are entitled to do so because there are no locks?