Why is it legal for China warships to enter Japanese territorial waters?

Harmless passage. The territorial sea is a part of the territory of coastal countries and belongs to the sovereignty of coastal countries. However, in the territorial sea of a country, foreign ships enjoy the right of innocent passage. "Passage" refers to navigation through the territorial sea for the following purposes: First, crossing the territorial sea without entering the internal waters or staying at berths or port facilities outside the internal waters. The second is to sail to or out of internal water or dock at such berths or port facilities. Adoption should be carried out without interruption and quickly. Passage includes suspension and anchoring, but only if it is incidental to normal navigation, or necessary due to force majeure or distress, or to rescue people, ships or aircraft in distress or distress. Conditions for the right of innocent passage: First, the passage of foreign ships through the territorial sea must be innocent. "Harmless" means not harming the peace, good order or security of coastal countries, nor violating the rules of international law. According to 1982 United Nations Convention on the Law of the Sea, acts endangering the peace, good order and security of coastal countries include: illegal use of force, military exercises, collection of defense information of coastal countries, propaganda acts that affect the security of coastal countries, taking off and landing aircraft on ships, launching or landing military devices, intentionally polluting the sea, illegally fishing, conducting research or measurement activities, and interfering with the communication system of coastal countries. Second, when foreign ships pass through a country's territorial waters, they should abide by the relevant laws and regulations of coastal countries, such as customs, finance, immigration, health, navigation safety, marine living resources conservation, environmental protection, scientific research and measurement.