Japanese intelligence is partnered with foreign agencies, which is generally seen in this business. This relationship becomes tighter, when they recognize the counterpart as allies to each other.
There is one uniqueness for Japanese intelligence, as they are one of the divisions at the police department, hence it is less likely to conduct the espionage outside the country. The budget basically does not allow to and the organization cannot send many officers outside the boundary. Some of them are actually sent abroad, though they are functioned as a liaison, not for spying.
In this situation, Japanese intelligence provides a domestic information trading with the foreign security intel, as its exchange is assumed to be a give-and-take, necessary to trade with something. However, Japanese security information is quite limited in terms of terrorism, especially for the reference to the outside world. As a result, they accept foreign agencies at the domestic cooperation, or they are required to leave some of the illegal activities untouched, conducted by foreigners.
The give-and-take itself is necessary at some point, but Japan as the country loses more than takes. Japanese intelligence even sells the people to foreign agencies, under the name of give-and-take. Foreign spies sometimes are out of litigations, even though they killed Japanese citizens. It is originally executed to maintain the public security, but as a reality, the Japanese security is totally breached.
This is because Japanese intelligence believes some of the sacrifices are inevitable to maintain the security, which is why some of the murders are untouched. This outcome is totally nonsense and it is quite unsure who they would like to secure from whom. They sometimes have claimed there is no way to contain the foreign espionage as there is no law to oversee, but it is not necessarily true. It is quite common to capture the foreign spies with petty crimes in other countries, which is one of the ways to contain their conspiracy. The main espionage issue in Japan is related to North Korea, though it is partially because the issue at CIA is escaped from the public eye with this trade and their manipulation.
The police department and its intelligence restrict their countermeasure against foreign espionages, partially due to the give-and-take and partially due to observing their organizational activity. However, it is totally the same to leave them free if its observation has continued for decades without any conduct, while the intelligence justifies their inaction, claiming they are under surveillance. In this way, they can fulfill their job obligation.
This behavior is sometimes ended up with the disaster. The assassination sometimes occurs in Japan, but it seems nearly an accident if successfully organized, hard to pursue the real responsibility behind the operation. Although, the police knows sometimes that it is not just an accident. Nevertheless, it is only prosecuted when the assassination plan is precisely revealed. In this sense, their claim makes sense that they cannot prevent the assassination, as there is no law to capture them, just being a spy.
Having said that, it is also a result that they leave foreign intelligence free in Japan. They know the spy is out there, who sometimes possesses a capability to assassinate, though they claim that they cannot uncover the plot totally, to capture them. However, their assertion is just wrong and they can restrain the activity of foreign espionage in Japan in the legal way. It is harder than to make an excuse, but it cannot be impossible. If impossible, there is no reason to maintain Japanese intelligence. If the give-and-take works as they claim, they just ask the criminal spies to their custody, who is in charge killing many people. This kind of trade is everywhere seen in the world.
The real issue is that the foreign intelligence represented by CIA is totally untouched by the police and its intelligence. If their activity is no harm to the Japanese citizen, that should be acceptable, but there are many cases the freedom of the people was infringed by their activities. This is not the only issue but there are many political conspiracies plotted by them, including the assassination.
One of the main reason to unleash them is that Japanese intelligence is actually at the side to plot the conspiracy including the assassination, not to stop them, even cooperating with the foreign intelligence. This kind of operation is also seen in the US, in Europe and in everywhere in the world.