it was classified as a law because there is scientific proof that this is true. it proves true every time.
A scientific theory refers to an in depth explanation of a specific observed phenomenon while a scientific law refers to a statement about a particular observed phenomenon. Scientific laws are usually presented in form of mathematical expressions for the observed phenomenon; examples of these are Newton's law of gravity, Mendel's law of independent assortment, etc. Scientific law basically describes the observations that are made and these observations are usually similar to other related observations.
