I think you are confusing harm reduction with incentives.
Is the following an incentive or harm reduction. On friday night, the kid says I am going to hang out with my friends (And you know that means activities including smoking pot) or I am going to watch TV (Or Netflix or whatever). Sitting at the shabbos singing zemiros is not an option you are given and in fact if you enforce that choice, they might leave without asking you to go hang with their friends smoking a joint
Which do you choose? Is it really an incetive to let them watch TV (Or Netflix on their ipad with 4g that you cannot control with the router) or harm reduction