At the moment DDG is effectively a customer, not a competitor. If DDG ever became large enough to show up on Bing's radar (Bing currently has 600x as much traffic), you can bet that the terms would change.
http://developer.yahoo.net/blog/archives/2010/08/api_updates...
"We are exploring a potential fee-based structure as well as ad-revenue models that will enable BOSS developers to monetize their offerings. When we roll out these changes, BOSS will no longer be a free service to developers."