Paramount Skydance said no to airing a 30-second ad from the Freedom of the Press Foundation during Sunday's UFC stream on Paramount+.
The ad called out CBS' billionaire owners for hurting press freedom and raised concerns about their ties to Donald Trump.
Paramount called it a "conflict of interest" and blocked it.
Seth Stern criticizes Paramount decision
Seth Stern, chief of advocacy for the Freedom of the Press Foundation, criticized Paramount's decision, saying it reflected poorly on press freedom and public trust.
He stressed this isn't just about one ad, but a bigger trend where media giants put their interests first.
The group plans to share the ad online as they continue to oppose the merger, which got US approval on June 12 but still faces hurdles abroad.