Right now they are ‘free’ but it is clear what the path forward is:

4.3 Advertising. The Service currently does not include advertising in the maps images. However, Google reserves the right to include advertising in the maps images provided to you through the Service, but will provide you with ninety (90) days notice prior to the commencement of advertising in the maps images. Such notice may be provided on relevant Google websites, including but not limited to the Google Geo Developers Blog and the Google Maps API Group (or such successor URLs that Google may designate from time to time). During that 90 day period, you may terminate your use of the Service, or provide notice of your refusal to accept advertising in the maps images in accordance with Google's policies and procedures for providing such notice (which Google may make available from time to time in its sole discretion).

Lawrence Ricci
www.EmbeddedInsider.com