There is a significant difference between a large-scale conduit for the speech of others (Twitter) and a local bakery or embroidery shop. I think the law should treat large-scale places of public accomodation differently, and there's plenty of precedent for that. The action in question is different, too. A baker who refuses to creatively express certain messages isn't muzzling anyone. And as a mere conduit, Twitter isn't speaking when they host our messages.
See Tweet