On another post I'd discussed the dangers of cheating on bulk oil, telling a story about some co-workers who had run a theatre popcorn scam by reusing old boxes, scraping the gum off, etc. The theatre charged them for boxes used.
I guess someone else could do the same thing with a filter, in addition to not having to do the work, a filter could be charged and then re-used or returned by (a friend of) the tech.