I'd say it's the opposite: people go for the movie, they don't give two damns about the "bonus content" (who does anyway?), so they need to be force-fed with it, just like with ads, if they're to see it. As for why do it in the first place, I'd say that "old movie + exclusive premiere of new extra content" may lead some people to choose the cinema over countless other ways they could watch the same movie at home.

In that case wouldn't it make more sense to put the featurette after the film, so people will stick around through the old movie to see the new hotness?

I don't see why it would. As the author said, featurettes are traditionally shown before the movie & this was clearly advertised. This gives movie goers a chance to ask about it if it is that important to them. That seems like there's enough inertia for people to expect it to be before. Theaters also want people to show up early before a movie's start time so there's a higher chance they get hungry or thirsty while they're there and they also want people out of the theater asap after the movie so they can turn the theater over for the next showing or go home. If they get to start early because people are leaving during the credits, even better.

"The solution is simple: preserve the wonder for first-timers by putting these featurettes AFTER the movie. Tease it before the feature."