Your argument was lost a hundred years ago or more. There are many regulations on businesses. On their speech, on their pricing, on the products, on hiring practices. The list is quite large.
AT&T can't restrict who can use their network to make phone calls based on the political views or the content of the call. Why should Google, Facebook, or Twitter? Especially when special liability exemptions were made without which they likely wouldn't even be in business.
I mean the simple answer is that I think AT&T should be able to restrict their network based on those criteria. If the people of the US don't like that, they should vote to either build out equal infrastructure to replicate that functionality publicly, or vote to buy out AT&T's infrastructure.
Or we can cancel all their easements, remove their right to run fiber and copper over public lands, and not renew their radio spectrum leases. These networks exist because the public gave them privileges in order to build the networks. Without those privileges they wouldn't exist.
Plus we could make them civilly and criminally liable for every bit of fraud, defamation, child porn, copyright violation, and etc. that involves their network.
AT&T can't restrict who can use their network to make phone calls based on the political views or the content of the call. Why should Google, Facebook, or Twitter? Especially when special liability exemptions were made without which they likely wouldn't even be in business.