Did the US dentist go to school longer? As I said, I'm sure they paid more. Is the US dentist more skilled at their job? Are the requirements to practice dentistry lower in India? Are the results better or worse in India?
You also mentioned the lower wealth in India. There is also a lower standard of living cost. Costs for products and services have long-since been decoupled from expenses and people charge what they think the market will bear.
Also, is there a greater supply of dentists? My understanding is that the AMA artificially limits the supply of medical doctors, but I'm not sure about dentistry. I also have no idea what it's like in India.
Given what little I know of the situation, I suspect I'd support the idea of Indian dentists making more money, not US dentists making less money. I believe teachers and medical professionals should be well paid. $150,000/year isn't that much. We have programmers who make larger salaries, and they don't even give me a free toothbrush.
Edit: Google tells me that the average salary in India is $616, so the average Indian dentist makes about 8.5x the average income. That's about $225,000 in the US.