I don't think you know what a scam is. It sounds more to me you're adding more to the scenario so you aren't wrong. Unless you're somehow suggesting it was a double double triple dog dare ultimate challenge aggro krag scam.

If you see a "No Trespassing" sign, it means no trespassing.

If you see a "Slippery Floor" sign, it means the floor is wet.

If you see a "Red Light Camera" ahead sign, it means there's a red light camera ahead.

What seems odd to me, is that after seeing a "I will rob you" sign, there are people out there who would experiment to see if it's even true. And when they realize the sign is truthful, somehow they can cry it was a scam all along? No, they didn't read the sign. That specific burden falls on the "friend".