...except the game mentions that they are all ghosts that E. Gadd had previously captured. With the Poltergust, presumably. And then changed into portraits. And then freed by King Boo.
...have you even played Luigi's Mansion?
Could you be any less respectful?
Of course I've played it. My point is that the ghosts expressed no self-awareness; they offer very little in dialogue to suggest they were. To say their motivation is to avoid being captured is no different than applying that to the ghosts of Dark Moon--who we also know are also being put to task by King Boo.
I see no meaningful distinction between the presentation of the two game's ghost motivation.