Why does this difference between art and science justify art's comparatively hostile attitude toward "derivative" works?Because science is engaged in a search for truth, new actionable information. Art is expressive, so copying someone else's work (for display rather than to develop one's technique,of course) is a sort of freeloading.
It's also worth considering the economic incentives for science and art are also pretty different. For good or ill, there are few scientists who labor unrewarded out of compulsion, not least because there isn't a whole lot of science you can do without a budget, although I can think of a few mathematicians who fit that bill, eg Grigori Perelman. However, by mentioning the economic side of things I wouldn't want to give the impression that I support existing copyright regimes, which I think are actually economically destructive to individual artists in many respects - it's just that I don't support the opposite position either, of nothing being truly original and therefore nothing being deserving of copyright protection.
I do think testability, or rather falsifiability, is important in science insofar as it makes the differences between conjecture and theorem. But I see where you're coming from; there can be a lot of value in an elegant theory even if it turns out to be superficially wrong, because it can open up new vistas of intellectual inquiry - Riemannian geometry springs to mind, where the failure of [many previous mathematicians] to prove Euclid's parallel postulate by reductio ad absurdum opened up a whole class of non-Euclidean geometrical perspectives even though such geometries were 'wrong' in the Cartesian context.
But to get back to the main point, with science your aiming for results of some sort, whose value lies in their explanatory power (even if that opens up a host of new questions, like some empirical observation that points out a natural phenomenon does not behave as we thought it did, without offering any theory as to why that should be the case). Art, by contrast, is inherently subjective, and might as easily be constructed to alienate or frustrate analysis, or be indifferent to the possibility thereof.
I think there's a tendency today to see art as an accretion of communicatory techniques, but I'm deeply at odds with this view, and feel that it reduces artistic endeavor to the level of marketing. (This is a very personal perspective, obviously.) If you're working creatively you're often asked/advised 'tell a story' or 'articulate what you want to say' or 'identify your point of view'. In a postmodern world, art is treated as a lens through which to view the artist, and there's a great emphasis on context and explanation, such that 'artist's statements' are considered essential to the presentation of art. To my mind this is an awful state of affairs, reducing art from the trancendental to the transactional. Self-consciously derivative and combinatorial art exacerbates this.