That's not actually slippery slope fallacy. He's simply postulating that event A would lead to event B.
Or in clearer terms, his hypothesis is that rewarding undesirable behavior will encourage that undesirable behavior in the future.
That rewarding of "undesirable behavior" encouraging future "undesirable behavior" is one of the major reasons why gambling is such a huge business. Just look at the number of casinos, lotteries, sports betting parlors, poker rooms, etc.