Well Apple isn't exactly known for giving things away for free, so putting Yelp as the top-tier result in Apple Maps is a pretty good indicator that a deal was made.
I wouldn't mind it if it didn't force me to install the Yelp app to see more details/photos. They must have some kind of deal.
Apple is rolling out their own reviews system so I expect Yelp and the other sources to be phased out in the coming years.
I dearly hope so. It's the only thing I don't like about Apple Maps.
It seems to be at least partially content-related, because I've seen both Yelp and TripAdvisor in Apple Maps.