I think that the issue is that the vast majority of Active & Performance line batteries will be under warranty & the market is not yet big enough for none branded replacements.Is it not possible to buy none branded versions of these batteries for a fraction of the cost of the branded product, or is it the case that the market is still quite niche.
Will probably change in a few years as both the number of e-bikes around & the number of those bikes out of warranty grow, both factors giving a bigger market for a third party to get involved.