Just for clarity, PPBM only applies to international flights. UA discourages voluntary separation but it's not a hard line -- similarly someone who misses a connection isn't (typically) considered a voluntary separation and the bag may still fly. UA has gotten so good at this that there have been several times in the past year where on landing I've gotten a "Your bag arrived on an earlier flight! Please see the baggage service office..." text (a few misconnects but most of those the bag got loaded on an earlier flight than I booked, either because that earlier flight was not a legal connection for booking or because I booked something else, e.g. to avoid a CRJ200 on what would have been the first legal connection)

From what I've seen airport authorities also are much less worried about baggage with clear owner identification (e.g. name and contact number) -- a bag with no tag definitely arises more suspicion faster, while a bag with a tag might get a airport police officer calling your phone to tell you you're an idiot and not much more.

I would not read any of this as me encouraging leaving baggage unattended, however.