She didn't tell them join a union. All she did was informing people that they have a legal right to organize.

In the US. Decidedly not in other countries, where it is just something you do.

That's certainly not the case, not even close. Unions are regulated by law almost everywhere which makes them an inherently political issue.

But if you look at, say, the UK, then unions actually fund one of the major political parties and have tremendous influence over it. Unions have in the past managed to topple elected governments. Beating the unions was Margaret Thatcher's flagship effort when she was first elected. And union membership has been in steady decline.

It's certainly not a USA specific thing. Union/management wars in the UK got very nasty indeed.