From my perspective these are two different questions.

Why is a paint splatter considered art?

Why is graffiti considered vandalism?

Understanding why ‘paint splatter’ is considered art necessitates some understanding of abstract art and the movement of Abstract Expressionism within Art History. During the early 1900’s – 1960’s, as technological advances (including cameras and photography), a wider range of industrial processes, and other cultural changes within many Western nations occurred, the way that artists understood and experienced the world began to change, as did the artwork that was being created. These cultural changes enabled a wide range of perspective changes within the art field as to what was accepted as valuable, necessary, and of interest. ‘Paint splatter’, was specifically a technique that arose from the movement of Abstract Expressionism. This is a rather complicated topic, that is deep, and has many facets, While I do think I could more forcibly argue the point I am not interested in doing so in this format.

Graffiti is, from my perspective, not questioned as a viable and worthwhile form of artistic expression. Graffiti is only considered vandalism when people make purposeful, visible marks on surfaces that they do not own or have permission to make marks on. To me the disregard of graffiti is less about its form and more about its disregard of the rights of property. To address the problems with graffiti a larger discussion about public vs private property is probably necessary.

In short, I do not think that the there is any question about the validity of graffiti as an art form, instead the problems that graffiti faces is where people choose to place this art.