I just got off the phone with a friend who is a manager with Customs and Immigration or whatever the name is now. I read him the initial post. Here is his answer. Once it has been established that you have the right to enter Canada with the proper documentation, the questions asked are not pertinent. Who paid for the ticket and what your profession is are not pertinent questions for determining customs requirements for duty. He however recognizes that refusal to answer the questions may flag you for secondary inspection. If this line of questioning is a problem in YVR, an observation can be raised to the Canada Customs Manager of Air Traffic Operations in YVR.
I did query him why I was asked for my ticket and he said that it was to verify my statement that I was in SIN for a social visit. The agent asked a question and watched my response. Since my response was prompt and credible, the inspection was over.