Retirees have less of a concern of future, hence they are less likely to abuse drugs. Pensioners also have the necessary income, and will not suffer income-related problems.

What's in it for them?

I think you've hit the nail on the head here, and it's not even limited to retirees: Why would anyone want such a job?

One might get someone unemployed and with no other choice to do it for a while, until another job comes by, but I don't really think anyone wants to do this for a living.