yeah I'm actually researching quite a bit on BHP atm (from doing the UBS banking challenge), but anyways here's a few points to think about:
1. there is a large growing demand in china for ores, and BHP is undergoing expansion in order to satisfy these high demands and they need funds to do this.
2. the main reason they're issuing a ton of bonds in america to borrow is because the AUD is currently quite strong relative to the US, and there's also speculation that it'll get even stronger, so this is actually making the cost of borrowing cheaper for BHP.
3. american investors on the other hand want to invest in aussie securities, this is because again the AUD is quite strong and it's a safe haven for US investors. Furthermore, as most american investor's atm are quite risk averse, there's a flight to quality to AUD denominated securities.
4. also if you examine BHP stock prices atm, it's continually increasing, suggesting again that it's quite a safe company to invest and hold funds in, whereas due to the economic unrest in America, investing in BHP is a much safer option than some of their local companies.