Originally Posted by X-Maniac
I think the imprisonment is a way of explaining what Magneto has been doing in the time between 1973 and the original trilogy. It justifies why he has not been an active menace in that period.
When we all saw X1, it always appeared that this was Magneto's first major appearance as a villain. But we know from First Class that he has been an active threat for far longer. Periods of imprisonment explain why he wasn't out and about causing mayhem.
This is what I think too. If it was a secret trial, it would explain why he wasn't well known at the time of X1. However it does explain why the government had a prison ready for him. He was a fugitive for the assassination of JFK. But that doesn't explain why he would be working with Mystique who set him up...
It's possible his entire involvement in X1 was kept under wrap to hide the fact that he had been able to escape from them in the first place..Which is why it seems like he hasn't left the prison at the time of the interview.