Like I said, it's worth more to have lots of free advertising being produced than going after the licenses of the advertisers.
The reason behind verifying would of course be that they thought that a significant part of the users did not have a license. Enforcing it would then force part of that theoretically large number of producers to focus on something else. More likely would be offering free licenses to content producers, something they are already doing!
Of course, they COULD enforce it, especially at places like Pond5, where sellers must upload official identification. Evidently, they do not think it's worth it.