The 'proof' that they can all be solved by logic only, is simply that they (the pappacom/times puzzles) are generated step by step by a program that does not use any trial and error methods. The program ensures that each step is a logical deduction from the previous clues only, and also does a "reasonable" job of grading them.
It also makes certain that they are symetrical and have only one soloution, both requirements of form for the traditional Japanese puzzle.
Hope that answers some questions, more is available fromt the times online pages,
CJ