Bill, these were my questions. Here's the answers I found, although somewhat unsatisfying:
Lennon was murdered one month before Reagan took office;
Lennon had been silent for five years raising Sean, but was making a comeback in 1980.
Lennon had been on Nixon's enemies list, was vilified for his Happy Xmas/War is Over billboards.
The government's repeated efforts to deport him failed, and he was granted permanent residency.
Appears he was deemed a threat when he resurfaced in 1980 and was here to stay.
Apparently the national security apparatus did not want him around writing songs and buying billboards during the new administration's efforts in illegal clandestine activity (Iran-Contra, etc.).
I plan to get David's new book.