Some newspaper ads are bought and paid for by a person or a group of people in order to voice a particular opinion or political point of view. Many times, the supporters will sign (have their name listed at the end of the ad in order to publically show how strongly they feel about an issue). Weissman has signed his name in support of the highly critical ad about Kennedy.

He sold the ad that was running in the paper. He had 'signed' the client whose ad it was, or had gotten the client to sign a contract with his newspaper to run their ad.