In no game but Oblivion are you a regular prisoner, in the first you are wrongly imprisoned due to being loyal to the emporer, the second you aren't a prisoner, the third you are imprisoned and sent to Vvardenfell for being a candidate for the Nerevarine. It is only in Arena and Oblivion that you are in Oblivion, and only in Oblivion that you are a 'regular prisoner', imprisonement is the tradition not being in prison, which is what Todd is forcing as a tradition, I have no problem being in prison in Skyrim, but I feel it should be because Bethesda feel it best serves the main story, not because it is a 'tradition'
In Arena, you are a Prisoner, it doesn't matter if it's wrongful imprisonment, Oblivion could also be wrongful imprisonment, but it leaves that up to you to decide. In morrowind you are a prisoner, it says nothing about why you are imprisoned, in fact in the coded package it only tells that you were
released from prison because you had the appearance of meeting the conditions, meaning you were already in prison. Even in the intro movie, it tells that you are a prisoner born of uncertain parents, that you were taken from the imperial city prison. There is nothing about why you were there in the first place.
In daggerfall, you could have various backgrounds, including one which have you be a prisoner which is released by the emperor. So we have 3 games where you are a prisoner, and one where you could be one.
And this was recognized as a tradition by many, before Todd said anything, he didn't force anything.
On Topic:
Shadowmere has red eyes, giving evidence of a supernatural presence, and she was unimportant to any quests whatsoever, she was unessential, yet was made immortal, this gives evidence that she was immortal not out of necessity, but simply because she was just an immortal being.