I came here to post the same thing. Besides the white supremacist propaganda, conspiracy theories, and COVID-19 misinformation mentioned by the OP, OAN (which Reuters reports gets 90% of its funding from AT&T) has consistently supported the 1/6 insurrectionists, with one of its hosts reportedly calling for mass executions of Americans based on false claims of election fraud.
Furthermore, AT&T regularly makes sizable donations to politicians promoting regressive and authoritarian agendas. For example, according to reporting about Texas's recently enacted near-total abortion ban, "AT&T was the single largest corporate donor to Texas lawmakers who sponsored or cosponsored Senate Bill 8." For another example, AT&T's donations are enabling widespread voter suppression efforts, not only in its home state of Texas but nationwide; in a study titled "The Corporate Sponsors of Voter Suppression" published earlier this year, the advocacy group Public Citizen found that AT&T topped the list.
Like the OP, I am a long time satisfied customer of Sonic. I'm happy to be able to patronize a locally owned business that provides great service. But I am extremely uncomfortable with any portion of my monthly subscriber fees being used to subsidize a political agenda aimed at restricting abortion rights, promoting COVID disinformation resulting in more needless deaths, increasing voter suppression, encouraging political violence, and undermining our democracy.
I echo the OP in urging Sonic to let your users know your position on this and what you plan to do about it.
 https://www.reuters.com/investigates/sp ... erica-att/
 https://www.thedailybeast.com/oan-goes- ... tion-fraud
 https://www.sacurrent.com/sanantonio/at ... d=27081538
 https://www.dallasobserver.com/news/ata ... s-12059128
 https://www.citizen.org/article/corpora ... 0-million/