McCartney's Japan drug bust.Why was he released so soon?

On January the 16th 1980, as Paul, Linda and their 4 children were making their way through customs at Tokyo's Narita airport, a custom official discovered 219 grams of marijuana concealed in Paul's luggage and also on one of his children. 

McCartney was arrested, questioned and then taken to the Tokyo Narcotics Detention Center where he was further interrogated.

McCartney was in Japan to give 5 concerts in Tokyo, Nagoya and Osaka with his band Wings. Subsequently all tour dates were cancelled leading to a compensation bill of £1 million for McCartney to pay.

Within a few days the 3 remaining band members, Denny Laine, Laurance Juber and Steve Holley returned to the UK harboring bad feelings towards McCartney for the loss of their income and the wasting of their time.

While in detention, McCartney was visited by the British vice-consul, who told the former Beatle that under Japanese law he was facing up to a 7 year prison sentence. However, after spending just 10 days in custody he was miraculously released and deported. 
How was that possible?
What really happened to facilitate such a premature release from a country whose legal system (even today) makes no exceptions...regardless of who, or how famous, you might be?

Below...German journalist: 
"How did you get out?"

McCartney: "I don't know!"

At around 4:47 in the below video McCartney is asked by a German journalist how he got out [of custody] to which he

finally answers... 
"I don't know." 

Why did he say he didn't know
when it's impossible he could not have known the reason for being released prematurely?

The first time Paul McCartney was fingerprinted dates back to the 29^th of November 1960 when he and the original Beatles’ drummer, Pete Best, were arrested on suspicion of arson in Hamburg, Germany. They were released and deported the following day. [Up to this point the story is verifiable] The rest of the story goes that when the Japanese authorities received the copy of Paul McCartney’s prints [which is the usual procedure] from Interpol taken at the time of his previous arrest in 1960 it was soon realized that they did not match the ones taken from the ‘McCartney’ whom they had in their custody. 

It is not known why he was allowed to leave the country on the 10^th day of his incarceration when, in accordance with Japanese law, he should have been sentenced up to 7 years in prison for the amount of cannabis he was caught with. It is purported that an official representative from the British Government (some speculate from MI5) arrived in Japan to speak with the Japanese authorities who were holding McCartney, soon after which McCartney was released and returned to the UK.

Maybe that's the reason why he said...he didn't know!