I didn't know that Di Modica still owns it. I suppose if he disagreed with what SHE has done, he could simply arrange to have the bull point the other way.
Just because Picasso (or some living author if you prefer for the example) doesn't own one of his paintings anymore, it's not artistically acceptable (arguably even culturally acceptable) for the owner of the painting to go there and just paint something over it that totally changes the meaning of the original painting.
Just because you are legally entitled to do something, it doesn't mean you should do something. As a recent example, just look at the United Airlines example for instance.
To me it represents a symbol in the war against men.
But what authority determines what is artistically acceptable, or which work's meaning is not allowed to be reinterpreted, commented upon or even ridiculed by another artist? What art permits in the service of social commentary and criticism is often at odds with what culture permits.
That depends on what you regard as equivalent to the canvas in the context of a public sculpture.
The actual physical form of the statue is probably the equivalent to its canvas - or I suppose it would also be the sidewalk underneath it, since that "contains" the artwork.
But to me it makes little sense to claim any arbitrary space around a statue as being equivalent to the artwork itself.