Mayflower,
This is simply not true. Yes poor people do sit on the floor but in the half-lotus posture.
The full lotus has profound physiological effects. In the parlance of yoga, there are two currents- male current and female current(i am omitting the sanskrit), sitting in the lotus posture, the male current is overcome, and by regulating the breath(inhalation equal to exhalation) the female current is overcome. When both the currents have been overcome, it will let you go into the meditative channel(sushumna, similar to the governor vessel).
There are subleties to the lotus posture itself- like putting the right foot on top, use of various mudras etc.
Blue_sky
