Why does Wand and Scroll Mastery (WSM) in the Spellsinger tree have a prereq of Wand heightening while it also gives the same DC boost?

This screenshot was taken 12/7/13 on Lamannia.
This prereq makes bards the only class with 6 or more levels of spells that need to spend 2 points per rank of WSM. Rogues get WSM in their Mechanic tree at 2 AP each rank. This I agree with as rogues have no spellcasting ability in their class and use UMD for all their wands and scrolls.

If you want to keep WSM with the DC boost and a separate Wand Heightening I would require WSM before Wand Heightening and Wand Heightening should cost 2 AP per rank. But this way is confusing, may be broken and make bards weaker than other classes with wands and scrolls where I feel that with both spellcasting and UMD they should be stronger than all other classes besides Artificers when using wands and scrolls.