From personal experience the deal they make with the host city is usually a win-win (for the municipality and for the citizens). The experience is great, JCDecaux gets to use some billboard locations, and the citizens get a dirt cheap public bike scheme. Here in Sweden it costs something like 8 euros per year. Yes, per year.
Another interesting thing is that they have a pretty OK API https://developer.jcdecaux.com/#/opendata/vls?page=getstarte...