Just to add to the discussion: for the same reasons as others give (the conditions applied and the fact that paralyzed does not cause you to go prone or drop items), the general disposition of a paralyzed creature is rigid.
However, it is a soft kind of rigid, similar to time-stopped bodies in a sci-fi show (think Hiro in Heroes or Quicksilver in X-Men) in that they do not fall over from loss of balance, and can be posed as readily as a Ken/Barbie doll (or, rather, some more thoroughly articulated doll).
However, unlike the time stopped examples, they are still aware of their surroundings (in real time). It is up to the DM as to whether they can move their eyes, but since they have no loss of awareness (including 360 degree visibility) it can probably be assumed that they can.
As others have explained it, they have simply lost control of their muscles - tensed muscles remain tensed, relaxed muscles remain relaxed, and the autonomous functions (heart, respiration, and balance) remain active, but conscious control of the muscles is lost. Since they automatically fail strength saves, posing them is not significantly hindered by the tensed/relaxed muscles (i.e. thy will change state easily due to outside influence).
I hope that this provides a better explanation of how a paralyzed creature can interact with (and be interacted with) by others/the environment.