RT is not bannd, but sanctioned due to being fully owned by the Russian government. The content they produce is not illegal, but US companies are not allowed to work with them. Stripe just wants to avoid problems and bsas them, due to Blinken claiming they are connected. That does not have to be true, but it pretty much gurantees legal trouble for Stripe, if they keep them as clients.

But also they are clearly professionals. Most employees having worked for Russian, Chinese, Iranian or Venezuelan state media is a clear sign. It also makes a direct connection to RT much more likely. So I get why Stripe wants to get far away from this.