Because you (or someone else in your position) can. Because it's a little bit of power that you have over the passenger in front of you ... and if you choose to exercise that power, there's frankly very little that the passenger can do about it.

To be fair ... this is hardly unique to the TSA. I knew a manager at a former employer who would make arbitrary decisions that caused a great deal of inconvenience to his subordinates, solely because it demonstrated that he was the boss.

I'm glad to see that you don't see the purpose in retaliatory screening; it shows that you're a nice person.