You are 100% completely and totally wrong.
The four major US carriers DO NOT block IMEI's for non-payment. All that happens is that the line gets disconnected and the account is sent to collections, and the owner is unable to unlock the phone for use on another network (official unlock, that is). They only block the IMEI and enter it in to the database if it has been reported by the owner as being stolen.
It's a rumor that has been floating around macrumors and a few other forums for years, and it makes the carriers look all big and evil, but it is JUST NOT TRUE.
So no, this phone didn't get on the IMEI blacklist because the previous owner didn't pay his bills
It's sad how you keep trying to pin such dark motives to whoever reported the phone as lost/stolen in order to rationalize not doing the right thing (turning it in to the police as a stolen item). Let's face the facts: it was almost certainly reported as stolen because...THE PHONE WAS STOLEN!
It's the same exact thing. Just because something is POSSIBLE does not mean you should assume it is the case, unless that possibility is an overwhelming likelihood. And in this case, we have NO reason to suspect insurance fraud.
You keep making points to discredit the previous owner of the phone who reported it as lost/stolen, which is sad and despicable. Just because there are a few rotten apples out there who will commit fraud like that doesn't mean that EVERYONE who reports their phone as lost/stolen should receive the same treatment.
I know one thing. If you had your phone stolen, and someone bought it from the thief and couldn't use it because you had the IMEI blacklisted, you would want the person to turn it over to the police so that it could be returned to you. Oh, you can try and act all laissez fair about it and say you wouldn't care, but we both know you would be angry if the person who bought it off the thief sold it again, knowing it was stolen property.
"Very frequently" means...well...exactly what it says. It means something happens often. The fact that you are confusing this with "more than 50% of the time" (ie. "most") is your misunderstanding.