Why do some triathlon seatposts go straight up and down instead of at an angle? Why is it that some triathlon bikes have a seat post that adjusts straight up and down instead of the more standard axis that aims at the bottom bracket?

This means that if you raise (or lower) the seat on the special triathlon bikes, the torso/leg angle would change more than for a normal bike, right?  Doesn't this also mean that the straight-up vertical adjustment of these special triathlon bikes do not fit as wide a range of riders as if the seatpost adjusted in a way that kept the same torso/leg angle along the axis?

This question is not about triathlon bike geometry or efficiency of peddling.  Rather it asks about why the displayed vertical seatpost adjustment axis is used instead of a more standard adjustment axis for seatpost.

Here is a tri bike with the vertical seatpost:



Here is a more standard bike:
To crudely simplify things, a triathlon/TT bike position is much the same as a road position, but basically &quot;rotated forward&quot;, so your arms rest atop the very-low-set bars. A consequence of this is, the seat ends up further forward.

(source)
Using Chris Froome's TT position as an example, noting the hip position versus the bottom-bracket position:

(source 1 and source 2 - I couldn't find a good side-on picture for the road-position, so it has quite a lot of skewing to align to the TT-position picture)
This &quot;Proper Bike Fit for Triathletes&quot; article has a vastly more detailed explanation (and is the source of the first diagram!)

Doesn't this also mean that the straight-up vertical adjustment of these special triathalon bikes do not fit as wide a range of riders as if the seatpost adjusted in a way that kept the same torso/leg angle along the axis?

As long as the frame is roughly the correct size, the tri bike should be just as adjustable as a road-bike, if not more..
Look at the tri bike image in your question - the saddle rails cover around 3/4 of the length of the saddle, allowing a greater range of movement forward/backwards (compared to the road bike, where the rail often covers less than half of the saddle)
Additionally, the aero-bars are much more adjustable than standard drop-bars (which are mostly limited to stem-length adjustments). Aero-bars can potentially be moved quite far forward or backwards (although at some point you will hinder steering/stability)