It is true, there is no established, universally accepted way to play the theremin. People generally regard this as a good thing, and confidently proceed to play the instrument in the way that seems best for themselves. No one would do this with a traditional instrument, like the violin or the oboe, but it is pretty much how everyone approaches the theremin.
If it were possible to make a living strictly as a professional thereminist, am estsblished technique for playing the instrument would likely have been developed long ago, and children would be brought up playing in a certain prescribed way.
As it is, there ain't never been no such animal as a "professional thereminist" and those who come to the instrument come as adults, with all their own preconceptions and ideas about how to play. If they have experience with other instruments, they will inevitably attempt to apply what they know to the theremin. This is to be expected because most thereminists are alone, learning in a bubble.
When Clara Rockmore began to study the theremin seriously (right after her career as a violinist abruptly ended) Lev Termen offered to build her a theremin whose pitch antenna was on the left side of the instrument, and volume on the right. This was because violinists control pitch with their left (fingerboard) hands. Clara decided against this, and proceeded to learn to play pitch with her right hand.
"I think it would be wonderful if everyone could play the theremin as well as I do, BUT THEY DON'T. I know that sounds arrogant." Clara Rockmore, the 'Gift Tape' interviews.
