If the seatpost were vertical and the saddle just sat on, there would be no tension. Because it is angled aft (like most seatposts, so increases effective top tube length as it is raised), weight on the saddle causes it to try to bend aft, so tension on the front of the seatpost and tube, and compression on the aft of both (cylindrical beam loaded in bending). Because the exposed section is nearly an order of magnitude longer than seatposts on large-wheel bikes (at least those with a level top tube), the bending moment from even a little aft force, can really matter in long term fatigue life. Dahon takes the approach of going large on seatpost diameter. Bike Friday goes more conventional diameter, but the long seatpost out of chrome-moly steel rather than aluminum.

Also note: In order for the seatpost to pass behind the bottom bracket when folding, the seatpost and tube angle are actually less inclined than on typical bikes. However, when deployed to normal saddle height, the saddle position relative to the bottom bracket position, results in geometry that is quite conventional; I refer to this as "effective/virtual seat tube angle", in the same manner as effective/virtual top tube length on conventional frames with a very sloping top tube.