I believe that "acts" are included in forms of expression. For instance, a protest march is an act. So is a ballet.
OK-
But marching generally only involves walking on a street - simple movement of one's own body, and ballet also only involves (more complex and certainly more beautiful) movements of one's own body, these are both fundamentally different than taking an object that is not a part of one's own body and changing it's composition.
They are both activities, certainly, but one involves acting upon an object, while the other is mere manipulation of one's own extremities.
Raising one's own finger or opening one's own mouth is different than intentionally destroying an external physical object.
I can see this difference as being the demarcation separating one act as protected expression and the other as a prosecutable activity.