But they're not. Acceptance of packages not addressed to you but to someone you've never heard of would have been the action establishing probable cause. 'Lead us not into temptation but deliver us from evil' is a standard for God, not the police. I am OK with law enforcement employing bait that is intended to be active to criminals; what they're not allowed to do is induce people to commit criminal acts who were not otherwise inclined to do so.
There's nothing wrong with accepting packages not addressed to you, especially from FedEx, what you're thinking of is 'opening someone's USPS mail' which most definitely is illegal.
If it was illegal to sign for a package addressed to someone else how come FedEx will let just about anyone sign for a package, even when you say they aren't home.
So in essence, you're saying it's ok for police to use evidence of doing nothing wrong as evidence of a crime?
There's a big difference between signing for someone who's not home and signing for someone that you know for a fact not to live there. It's not a criminal offense, but the standard for probable cause is one of suspicion rather than proof of criminality.
Well, to be precise, "reasonable suspicion" is a weaker standard than the "probable cause" required for a warrant. In either case, there needs to be suspicion of something specific, not just "this seems shady".
That said, you are certainly correct that "probable cause" is less than proof, and knowing nothing about the case it seems plausible that "this guy is accepting packages for people he doesn't know" firmed up some link in their reasoning and made the difference between lacking and having probable cause.
I don't think they were detained. That has a specific meaning of not being free to leave, where it seems here as if the police merely asked questions. The police don't need permission to ask you anything, although you're not obliged to answer either. However, we'll never know exactly how this went down legally speaking, since the top poster avers that he had his headphones on for the first several minutes of the interview.
In the specific example of a traffic stop, I believe you are considered to have been detained. You are correct that merely asking someone questions isn't the same.
eg. If you witness someone selling stolen goods, search them, don't sell them stolen goods and then search them.