Actually, you receive SSI in your home country just as if you were a US citizen. The rules don't actually change based upon your citizen (which now seems absurd since we are talking about people who illegally were working and paying into the system). The treaty was signed pre-9/11. It was mostly a formally and was done back when Bush was still optimistic that Fox wasn't going to be a complete tool.
I'm not in my native country. I'm here in CA and am now an American citizen. When I'm old enough and if there's still SS to draw I will receive a reduced American amount then it will be made to the full amount by my UK SS. But the minimum age is still 62 yrs old, not 55 yrs old. It's not a free giveaway as you were indicating.
http://www.goodolddogs3.com/HonorOurVets.html