Why do some triathlon seatposts go straight up and down instead of at an angle?
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)