It is not that simple. There's a continuum from writing a compiler to coding DestroyBaghdad. Usually people draw the line at "Is this tool primary purpose evil?".

This tool is in a (darkish-)gray area IMHO. I don't think that employer blocking "fuck" or racial slurs is necessarily evil but it's not that good either.

Automatic filters are pretty bad at doing their job, so filtering is a lame approach to solving anything.

A tool that can block discussion of unionization is certainly a grey area. When you literally advertise "blocking discussion of unionization", I feel like you've left that grey area, though.

Just to be clear, I wasn't saying it's not unethical. Just not illegal.

This is reminiscent of how Facebook used to allow people to microtarget job and housing ads to people based on protected classes like race, age, and gender. They ended up reaching a settlement and being forced to change their filter criteria due to civil rights laws. So I wouldn't be so sure that "merely providing the tool" is necessarily an entirely legal action.

They'll probably move to categories or something that can be banned, rather than explicit lists.

Kind of odd that they would choose to promote their product by touting a feature that is illegal to use.