Why is the hexose monophosphate shunt called a direct oxidative pathway?
This answer is based on a 2002 autobiographical account of the work of Bernard L. Horecker entitled The Pentose Phosphate Pathway. Here he refers to “direct oxidative pathway” but does not unambiguously explain this, so I shall summarize my interpretation and then provide evidence in terms of quotations.

Summary of my interpretion of Horecker’s account

When what I shall initially refer to as the hexose monophosphate shunt (HMS) was being elucidated in the late 1930s and the 1940s it was thought that its main purpose was the same as that was envisaged for aerobic glycolysis — to oxidize glucose to provide energy — and was merely an alternative way of doing this. The term ‘direct oxidative pathway’ was used to distinguish it from the glycolytic pathway (then refered to as the Embden–Meyerhof–Parnas pathway), presumably because six cycles of the oxidative phase e could completely oxidize one molecule of glucose 6-phosphate to carbon dioxide, whereas glycolysis only oxidized it to pyruvate. Presumably this was regarded as ‘indirect oxidation’ because complete oxidation required the further metabolism of pyruvate using the tricarboxylic acid cycle.

The term ‘direct oxidative pathway’ fell out of use as it was realized that the main functions of the pathway were to provide pentoses for nucleic acid synthesis and NADPH for synthesis of fatty acids, steroids, and reduction of oxidized glutathione. It was for this reason also that the name most commonly used for the pathway changed to the pentose phosphate pathway, transfering the emphasis to one of the products.

Quotations from Horecker 


  …Haas arrived from Warburg’s laboratory in 1939. Haas asked me to join
  him in the search for an enzyme that would catalyze the reduction of
  cytochrome c by reduced TPN (now NADP). This reaction was thought to
  be the missing link in the electron transport chain from substrate to
  oxygen and marked the beginning of my interest in what was then
  thought to function as a direct oxidative pathway for the metabolism
  of carbohydrate but is now known as the pentose phosphate pathway…
  
  …These observations gave rise to the hypothesis that the oxidative
  pathway was really a cyclic mechanism for the direct oxidation of
  carbohydrate. With each turn of the cycle one molecule of carbon
  dioxide would be produced, and the pentose phosphates formed would be
  metabolized back to hexose phosphates to start another cycle. Six
  turns of the cycle would result in the complete oxidation of one
  molecule of glucose…
  
  …The function(s) of the new
  pathway, however, turned out to be quite different from the pathway
  for the direct oxidation of carbohydrate that we had expected. It
  provides two mechanisms for the production of ribose 5-phosphate. One
  is the “oxidative branch” of the pathway, which also generates 2 eq of
  TPNH (NADPH). Ribose 5-phosphate can also be formed directly from
  hexose and triose phosphates by the non-oxidative rearrangements
  catalyzed by transketolase and transaldolase. Where large quantities
  of NADPH are required, as in the synthesis of fatty acids or sterols,
  the excess pentose phosphates produced would be recycled back to
  hexose monophosphates.


[My emphasis]

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