What is the justification behind the right of innocent passage through territorial waters, when the waters actually form a part of the states sovereign territory?

The rationale behind this controversial concession granted to foreign vessels when entering territorial waters is that prior to the 1982 Convention, all coastal states were granted only 3 NM from their coast as territorial waters. The Convention changed this to 12 NM. This meant that the traditional freedom of the High Seas was being curtailed, as every coastal state was adding 9 NM to their sovereign territory. As a compromise, the right to innocent passage was granted.