You make an excellent point on the license agreement conundrum, and will definitely blog further on this one (probably on my more business model focused blog: www.thenetworkgarden.com)
Back in 2002-3, when we did Verdada (http://www.verdada.com) and created our VSOS client software (the SOS stands for save, organize and share), the license limitations on Amazon Web Services technically precluded storing local copies of listings on client PCs, Google limited the number of search transactions, and the limitations on co-mingling content items from different services really seemed geared towards limiting the space to hobbyists.
It seems that not much has changed. I know Yahoo's APIs have similar licensing limitations.
Part of this is a gambit by the major sites to find the balance between being open, learning how open is advantageous and not wanting to disrupt existing business models and partnerships.
Will be interesting to see how all of this shakes out.