The incremental cost to Kodak of giving away 100' of movie film to an individual is almost entirely related to the cost of fulfilling the request. The film itself costs them almost nothing. In return they achieve a one-to-one contact with someone who at the very least will end up being favourably impressed with Kodak.

They would have to pay a lot more for other types of marketing capable of achieving the same result.

The fact that the OP intends to re-purpose the movie film doesn't really detract from that marketing benefit. The extra difficulty involved in the re-purposing effort also means that the availability of a free sample like this doesn't markedly reduce the market for the full price product intended for still photography.