By golly, for the first time ever I'm about to disagree with Bernhard!
It is ok to disagree with me.

Why should a pianist control their grimacing/humming/moving about? After all, music has the power to move us. It moves the emotions and makes you want to smile, laugh, cry. It can make you happy and make you want to dance. The rhythm can cause you to sway or tap your foot to the beat. If you are at one with the music you are playing you feel these sensations.
At home, perhaps. But in a public performance, a pianist histrionics makes it impossible for me to be moved by the music. And as a member of the public if I started clapping hands and stomping my foot every time I got over-excited I would come to the attention of securtity who would show me the way out, since my histrionics would be interfering with other people's enjoyment of the music.
Sure, this sort of body discontrol has its appeal to certain people. But then that is why rock concerts exhist. Go to one of them and dance away (who cares about the noise? sorry , I mean music?)
Sure, you can train yourself to remain perfectly immobile and impassive as the music flows through you. But isn't that unnatural? Sometimes when I play certain pieces I am so moved that tears well up in my eyes and course down my face. Should I train myself not to feel, just so a camera may not pick up an offending tear running down my face?
But why stop at the tears? why not sob uncontrollably and cry "mummy, mummy!", and throw yourself in the floor

Surely you will agree with me that such behaviour would not be conducive to music appreciation by the audience (and I repeat again, in the privacy of one's own home anything goes).
However it is worse than that. Most of the absurd movements of pianists are not natural at all. They are carefully rehearsed and many times the pianist is coached on them. Brendel is on record saying that the first time he saw himself on video he was shocked at his natural grimacing, because he felt it was out of context with the music. So instead of stop grimacing, he started practising with a mirror so that he could match the grimaces to the feeling of the music. Unnatural enough for you? Then take Helene Grimaud. I have a video of her playing Rach 2, where her movements are so obviously rehearsed and so hamly delivered that it is painful to watch. By the time you get over it you realise that you have not heard a single note she played. (She sways, she looks up dreamly, she shakes, and it is all fake).
Then another day a friend showed me a video of some opera-singer (I do not know his name - a tall, shaven-head, glass wearing afro-American with a wonderful voice). The video was sort of cross over, and the singer was singing some drivel by Enrique Iglesias

. The problem was, for all his good singing, he had been clearly coached to smile and make "expressive" hand movements at certain spots in the music. It was completely ridiculous.
If one cannot stop moving and humming, then do what Richter did: Play in the dark.
Jazz pianists have always felt free to let their bodies express the feelings induced by the music. Most jazz pianists tap their feet as they play. Many hum along (Oscar Peterson, Keith Jarrett, for example). One extreme is the great Thelonious Monk who would actually get up and dance around the piano when so moved.
Well, jazz is not European Art Music. Somethings are appropriate in one genre and inappropriate in others. To each its own.
And there is something else too. The hidden statement here is : "Why can't classical music be more like jazz or pop music?" So consider this: "Why can't chess be more like tic-tac-toe? So much simpler, so much more fun, none of that stuffy seriousness of chess players!".
Why should we be so restrained when it comes to Western European Art Music, or so-called classical music?
Ultimately because the performer's histrionics distract from the music and throw attention on the performer.
Best wishes,
Bernhard.