Because if you fly the trip with a customer who "believes" that you are intoxicated and then voices their concerns to you there may be an issue. They can call any news outlet and give them all their facts on how they were so confident in their assumption and that after voicing their concern you continued to operate the flight.

At this point it will be too late for a test and you would pass anyway but I have a feeling it could cause you a headache or possible suspension pending investigation while it all gets sorted out. Companies do not take this kind of media coverage lightly and will take the necessary steps to ensure their flight was operated according to the rules.

My point being that it doesnt take much of a ripple to make a wave. Me personally, I will not mess around with this. If they have reason to believe I am intoxicated I will get tested to clear my name and then not allow them to fly on my flight (if I am not removed from the trip as a result of the testing delay).