For a short-term temp, I can uunderstand the Agency's policy, but for a temp who has been a valuable member for a year and a half it's ridiculous. The temp could possibly feel betrayed and it would not be great for an employer's reputation if valued employees are called up and told "Sorry you didn't get to say goodbye, but don't go to work on Monday."
It would come as a needless shock to someone who did a good job and would burn bridges if the employer had a suitable position in the future. I would rather stay on good terms with a hard-working, well-liked employee. The Agency's policy would sour that relationship.
I'm glad you went with your instinct to do the right thing!