Perhaps these people aren't worried about actively pissing people off. Maybe they're worried about someone misinterpreting something. Maybe they're just a bit concerned about advertising their home as being well-stocked with radio gear and other electronics.
Me personally, it's a mixture of all the above so I do utilize a PO box. Additionally, any ham radio stores or other junk mailers get my PO box address, which I never check. So there's a little less junk mail to sort through from my home mailbox.
If you think using a PO box for your FCC address is paranoid, does that mean it's paranoid to shred financial documents? Credit cards/credit card offers? Bills? Old checks? What about locking your door at night or when you leave the house? Locking your car door? Putting your car in the garage? Utilizing a safe deposit box at a bank?
If these people want to take additional steps to secure their lives, identity, and property, so be it. It's their decision. It's not a stupid decision to add layers of protection. This is coming from a person that has had his identity stolen. It's a new world we live in now, and you'll say that it won't happen to you until it does. People have a variety of reasons for wanting to protect themselves.