Well, stock footage is available for purchase to anyone. People who appear in it are one-time paid actors, not bound by any means to the product or cause the footage is used to promote, and also not bound to endorse it, and the footage/stills aren't for any one specific user. This specific footage has probably has any number of commercial users in the 8 years it has been available. Generally, footage such as what you saw here is bought for product sales commercials. The Clinton campaign bought usage of it -- nothing illegal in that, just ironic (and funny). They would have done better by shooting their own original footage.