I don't have a problem with a private company, such as YC or a production company, firing someone for publicly donating a massive amount of money to a political candidate.
McCarthyism was different because:
1. Many of the people who were fired, blacklisted, or persecuted were only suspected communists.
2. Somewhat related: if any of the blacklisted people were rich enough to do so, they didn't publicly donate their day's equivalent of $1.25M to any communist cause.
3. There is no threat of violence, prison, or losing his livelihood for Thiel in this case.
It would have been OK with me to fire (and only fire) them if they were actual communists. Political beliefs are not a protected class in the United States.
McCarthyism was different because:
1. Many of the people who were fired, blacklisted, or persecuted were only suspected communists.
2. Somewhat related: if any of the blacklisted people were rich enough to do so, they didn't publicly donate their day's equivalent of $1.25M to any communist cause.
3. There is no threat of violence, prison, or losing his livelihood for Thiel in this case.