Seeing as the plot can drive characters in a negative/regressive direction as a result of their choices in the face of circumstance, it would stand to reason that sometimes a Strength card simply does not make sense as the result of completing a Goal/Subplot. For instance, a character who sacrifices the good of their fellows for the sake of their own selfish goals is shown to be weak of heat, and that weakness should be reflected in the received cards.
The forcing of characters to increase in "power" as a result of completing Goals/Subplots also reinforces the idea that completing Obstacles with Strong outcomes means "winning" and creates the dynamic that Weak outcomes are bad and to be avoided where they oftentimes create the most dynamic and interesting plot developments.
By simply allowing the player (possibly for Subplots) or Narrator (in case of Goals) to choose what type of card is received as a result of playing the associated cards, Storium would reinforce that both and Weak and Strong outcomes and story developments are good and necessary for powerful storytelling, (which it appears to want to do by stating that both Strong and Weak outcomes to challenges allow players to "win control of the story").