He was trying to leave...so why deport him??
There might be a legal reason, such as a prohibition of deported individuals from re-entering the country for a set number of years.
The timing of this makes it related to the Jenkins case...which is the real issue at hand. The US must demand Fischer so they can demand Jenkins. Japan is showing that they are willing to honor the treaty with the US.
It will be interesting to see what negotiations go on for Jenkins in light of this event.