I think you need to push the fact that the company is not really doing marketing, but is arranging the sale of books up-front AND in such a way that the bulk purchase is not ignored by Neilsen BookScan (which compiles the bestseller lists and normally would ignore bulk purchases).
I think many elders and sometimes church communities are sort of invested in their pastor's writing career. They want to see their pastor do well, but they also want to share the fruits of their pastor's labour. i.e. The pastor, especially in a megachurch, writes a book and naturally many in the congregation will want to read their pastor's book and they think that doing a bulk buy will mean they get it cheaper. Sadly that bulk buy does nothing to get it onto the bestseller list and does nothing for the book's profile beyond it's niche audience in the pastor's congregation. So some promotional company comes up with a perfectly legal (though shady) scheme and the elders/deacons think it's a win-win! They pay a relatively small amount for 'marketing', the congregation gets the books they want at a discount and the church maybe even gets a cut of the royalties (some pastor-authors do formally sign over some of their royalties to their church in recognition for the fact the church doesn't have their pastor full-time), which could be substantial for a book with the label "NYT Bestseller!". The problem is the lack of transparency and the fact that as DanielJonceEvans pointed out: this could be embezzlement, not merely 'buying' a "Bestseller" label for a book.