To me it just seems like a lighthearted prank which resulted in giving flowers to someone who appears in need of cheering up. It's something that could easily have been on any comedy TV show of the last 30 years. And I think there's always been a monetary incentive; Candid Camera first aired in 1948.
But I'd imagine that Candid Camera required "participants" to sign a release. Same for those "Funniest Home Videos" and even stuff like Borat (and Tom Green, etc. before him).
The filming may be candid, but to air, the subject must sign a waiver. Whether the subjects were compensated for agreeing to the airing of the footage is a different Q.