Theremin Handedness?

As a violin learner, it is normal for me to control the pitch by left hand and control the volume by another hand.

So, it confused me that why most theremins I have seen on the video use the method different from other string instrument?

I play the theremin left handed (left pitch, right volume)... There is no rule on that at the theremin. You do what makes the most sense to you :)

"So, it confused me that why most theremins I have seen on the video use the method different from other string instrument?"

The answer to your question is that the theremin is not a string instrument.

Fortunately, the theremin can be configured as right or left-handed, so you can just make a choice and go with it.

Enjoy!

It is probably a good idea to play pitch with your dominant hand (i.e your left hand if you are left handed) as pitch requires finer control than volume.