No, that's not their assumption. The design goal is that we should buff others.
The problem is that Blizzard never really communicated this clearly. Thete for example encountered many Rets on Beta who were using the blessings wrong (all 3 on themselves or wisdom on healers and such shenanigans)
And all of us will see in pugs the same errors because many Ret players don't read guides or forum posts regarding this subject.