The alternative is that there's some fundamental reference frame. The obvious example is the theory of the aether I alluded to: light would propagate in different speeds in different reference frames.

More generally, you can't know that the laws of physics are the same in all reference frames until you actually do the experiments. If you think otherwise, you underestimate nature's ability to confound human intuition! A good example is is parity, which you'd think would be a good symmetry of nature, but is actually violated by some interactions: http://en.wikipedia.org/wiki/Parity_violation#Parity_violati...