Not all Map service is free, most of them need licenses, but they provide free using under some rules.
Here is example of Microsoft Silverlight maps control for Developers:
Developers at all types of organizations can use the Bing Maps Platform for free, under the following conditions:
* Use is on public-facing, non-password protected Web sites * Limit of 125,000 sessions or 500,000 transactions in a 12 month period
Developers using the Bing Maps Platform for free receive:
* Full choice of Bing Maps APIs * Aerial, road, and Streetside™ * Access to documentation and developer articles on MSDN * Access to the developer forum for support questions
Google, Microsoft, ArcGIS all need license fee if the software deploy and customer use. But free for development.
1: Bing Maps Licensing and Pricing Information: http://www.microsoft.com/maps/product/licensing.aspx