My reasoning is that sine you can feed those spirits POW, and since you can feed Ancestors POW, it means that you can feed spirits generally POW (how could you even give them POW for a Pact if you couldn't give them POW in the first place?). It's a great currency for making (expensive) deals with spirits, too.
And everyone can at the very least feed spirits POW as part of worship, so it makes sense to me that they can also feed spirits POW more generally, although this is less clear cut. (Further, since you can take POW from certain spirits, such as Disease Spirits...)
But anyway, off-topic! If your GM agrees that this works, it's a great sink for POW until your Allied Spirit is full, is all I'm saying.