There is actually no 100% fool proof pill available in the market. Most of them are just 97% safe and you still run a 3% chance of getting pregnant.
I think you are right. My doctor told me the same when I asked her about using one within 72 hours after unprotected sex as I wasn't ready for a baby yet. She told me to wait for my menstrual cycle to actually know if I had got pregnant.
I used a birth control pill and am right now pregnant with my baby due any time!