My explanation wasn't thin at all. I clearly explained the reason for imprisonment and even gave a similar use of such a plot beginning. .
Sorry, your explanation wasn't thin, just the argument.
"...it allows for better RP because you can imagine your character to be whatever you want before and after being in the prison."
This is the part I don't agree with. In fact I don't understand your point at all. How does it allow for better RP to
not be in charge of your character's history?
By starting in prison you're pretty much forced to base your background on being a criminal of some sort, guilty or not. That is, of course, unless we'll be a prisoner of war, which just starts us off with another preset background.
You also don't have any belongings to call your own or define you heritage, personality or origins (e.g. if you're Imperial you could start with an Imperial sort of armor or weapon or other gear). And your introduction to the main story line is set in stone. With the plethora of options and freedom provided by modern game engines, it just seems too easy to me to start us out as prisoners. Meh *shrug*