Freedom of speech and freedom of expression are rights that everyone has because they are human.
The first amendment is one protection of these rights that prevents the violation of these rights by the US government.
People still have freedom of speech and freedom of expression when dealing with non-government organizations. It’s a responsibility of these organization to respect these rights. Customers should refuse to do business with companies that do not.
Except that anti vaxxers are usually mentally ill themselves and are incapable of building product or service, particularly in the technology industry.
That's why we see a lot of alt-right or anti vax type people on platforms which consider their behavior hostile after it gets them bad press. Beforehand they usually turn a blind eye. It is an attempt to get out in front of regulation which will be inevitable if companies do nothing about it.
Also, not everyone on the internet lives in America, which means not everyone has freedom of speech.