Hot take, I see no issue with this. If you’re savvy enough to know about Tor and its purpose, you’re also savvy enough to know how to add a security exclusion in Defender. People who don’t know how to whitelist a program in Defender probably did not install Tor themselves and won’t be safe using a program with the capability to access the dark web.
It’s extra frustration for those trying to legitimately use Tor, but it’s also a safety check in the case of an unintended install.