I should mention that being a graduate student (which is the context of the original $16K figure) is not full-time employment. Typically, you are a full-time student with an additional part-time TA or RA position. Where I went, the graduate student stipend was technically payment for 20 hours per week of teaching or research. The rest of your time was considered as part of your schooling (whether you're spending that extra time attending classes and doing homework or just doing more research).Alex_Sanders will have to speak for himself, but he might have meant that $15K is very low. It does seem low to me. New Brunswick, Canada (where I live) has a legislated minimum wage of $9 per hour, regardless of education. This minimum wage is slated to rise to $10 per hour in less than a year. Consequently, anyone who has full-time, steady employment should make at least $20,000 per year, and university graduates could hope for a higher minimum annual wage.
If you add in a tuition waiver and health insurance, and divide by the 20 hours that you're technically being paid for, you are nowhere near running afoul of minimum wage laws.
Of course at the end of the day you're putting in more hours than a full-time job and taking home only a small amount of money, so perhaps it's not an important distinction. It's certainly different than other graduate degrees, however, where you acquire a large amount of debt instead of actually taking home a net amount.
Oh, and getting a physics PhD does not guarantee a 6-figure salary. I would say it's not even likely. I'm pretty sure you can be a full professor at a top university and make less than that.