Actually, only 72%,not 75%.
Other than that, what you have pointed out is very true.
With a few exceptions, disease is more prevalent and life expectancy shorter the lower one is in the SES hierarchy.
It just happens that most smokers(72%) are in the lower SES stratums.
I believe the median family income in this country is $50,000.
Doing the math for the earnings groups up to $49,999 per year, we find that 32.6 million adult smokers earn less than $50,000 per year.
32.6 million smokers is about 72% of the 45 million or so adult smokers in this country.