But the Principle of Inertia is itself one of the law of Physics. So essentially we are saying that the laws of Physics are true in frames where the laws of Physics are true. It is perhaps amazing that Physics, based on this tautology, works at all. But it does work very well.
You may wish to muse about the fact that post-1905 we can say that not only is uniform motion relative, but so is acceleration. If two observers are accelerating with respect to each other we can not say which one is "really" accelerating because there is no absolute frame to which we can compare the two observers' motions. However, we do know that if one of these observers is in an inertial frame of reference, then the other is not.
Considerations of relative acceleration is one topic of Einstein's General Theory of Relativity of 1916.
Aristotle: It is the nature of chickens to cross the road. Newton: Because no force caused the chicken's state of uniform road crossing to change. Einstein: Is the chicken crossing the road or is the road moving under the chicken?
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