Unlocking a phone

I bought a iPhone 6S, brand new that I'm trying to use international, the phone is unlocked in the USA no problem at all, but apple doesn't want to unlock international because I cant provide a proof of purchase, the phone was won at a raffle by previous owner and I cant get the proof of purchase, but the phone is clean, not stolen or under a contract with any carrier, I have contacted several "senior advisors" and they told me the same, they don't want to help, I asked them if they can provide information of their supervisor and they told me that they don't have supervisor or nobody above them, that they are on top of the line ( they must be apple CEO's i guess, or at least that's what they think) anyways finally that gave me an email address of their manager Matthew_Campos@apple.com who I emailed about 4 days ago and haven't heard from him, by the way the advisors told me that is up to him to answer customer emails, I guess he doesn't care about customers. I just don't get it why apple doesn't want me to unlock the phone if there's nothing wrong with the phone. I asked a question and they told me that they don't unlock the phone to anybody that calls and request that the phone can be stolen, or under a contract, but they acknowledge that the phone is unlocked in the USA, and if I put a SIM from any USA carrier it will work, whats the difference of getting it unlock international????Why do they want to limit you of what you can do with one of his products if is paid already???