Listen, there's no question that if their "script" was written by an etiquette expert, it would be phrased like it once was to me in Tampa "sir, I have to check your bag, it will only take a minute, can you come over here with me?" That WOULD be preferable, 100%. However, these airport security personnel are, oft times, underpaid, underqualified, undereducated and underwhelmed with any pressing need you may have to make you flight. SOME of them are like this. Not all. If you run into one who can't manage to be polite, what are your options?

I just asked a friend of mine who was in Customs before he moved over to A.T.F. I asked him "when you worked at JFK, if a passenger was asked to allow a bag of his to be inspected to the full satisfaction of airport security folks, could he say 'no' to such a search?"
"sure" he said "and he would be told to turn around and immediately leave the airport. If the person in question was lucky, he ~wouldn't~ have Port Authority Police following him for further questioning"
So you don't HAVE to consent to the search...