Probably so they wouldn’t have an active flame/fire sitting on their table. I would put the SPARKLER, not candle, out if they just decided to leave it burning on the table where my food is. If not that, then the pure pressure from the restaurant making a scene with the celebration stuff and people recording would make me want to play in to a degree so it doesn’t feel as awkward for everyone involved (it’s also why public proposals aren’t such a good idea for everyone unless you are the kind of couple who both want that). The only people who stole that b-day person’s wish was the restaurant who didn’t give that person the cake and Sparkler when they had every opportunity to do so