By that definition, SWF would apply to Heavy Repeating Crossbows and so forth.
The truth is that the "Single Weapon Fighting" feats are misnamed; what they really mean is Single Handed Melee, where you have a weapon in your right hand and your left hand has nothing to do with melee. By choosing the wrong name for the feat series, the developers got some players confused about how it should work.
By that definition, anyone could build a level 1 character who wields a Longsword and Heavy Shield while benefiting from the SWF feat. But that's not what it means.
It would be bad for the game if S&B style is a 6-feat investment while TWF, THF, and SWF are all 3-feat investments.
- It is an important design goal that fighting with a shield be a reasonable choice, and fighting S&B should not be entirely dependent upon having the Vanguard tree from Fighter or Paladin. Therefore there must be feats available to transform S&B from a bad choice to a good choice.
- Given that 3 feats is enough to change TWF or SWF from a bad choice into a good choice, 3 feats should also suffice to make S&B into a good choice. That is especially important because one of the classes intended to use S&B has 11 bonus feats, while the others have none. Spending 3 more feats to get GSWF alongside S&B style is easy for a Fighter, but really rough on a Paladin, Cleric, Favored Soul, or Sorcerer.
- If the updated Shield Mastery and Shield Bashing feats are made strong enough so that they're a good choice on their own, then it will be overpowered to allow SWF to work too. Alternatively, if the updated Shield Mastery and Shield Bashing feats are so weak that they need SWF to make them worthwhile, then the feat cost will be unfair compared to TWF, THF, and real SWF.