And, like many rules the FAA makes, there is debate over whether they have the legal authority or physical power to enforce them. That's why they took Trappy to NTSB court rather than arresting him. http://www.forbes.com/sites/johngoglia/2014/04/12/faa-files-...
- flown a sufficient distance from populated areas
- should be kept within visual line of sight of the operator
Source: https://www.faa.gov/uas/faq/