It's never shown that the imperials have any loyalty to a cause or something. It's a typical dictatorship, it's all fear of Vader, The Emperor and Death Stars, which is over. In lots of countries dictators get killed and everybody who wasn't a friend or relative goes right along with the new guys. It was never a religious regime of sworn fanatics or anything, the imperials are all just weasels and bureaucrats who don't want to get choked to death. The rebels weren't outside invaders, it was just a government and still is. The senators were only kicked out 5 years earlier.