At least with a pool, you'd want them to be careful. It is a slippery surface, and a fall could easily incapacitate an EMT, or destroy life-saving equipment.

Edit: As for walking in general, it could also be because they wouldn't want a crowd to form which could obstruct them should they need to evacuate you more quickly.

T(walking_now) + T(walking_later) < T(running_now) + T(moving_through_crowd_later)