usually governments just tax or tariff the corporations directly instead of relying on their citizens to extort them physically

What if there are countries where the governments are the people

It’s rare, which is why this audience likely doesn’t understand it vs “you’re not allowed to do that!” You absolutely can do whatever citizens collectively decide, including a general strike or blocking ports.

Not every country has elected to be a slave to rugged capitalism. A surprise to be sure, but a welcome one.

The employees of companies are also people.

Indeed they are.

The unions negotiate fair indutry-wide rates and conditions for the members, not taxes.

Think of it like people having talked about their salary in advance so when they walk into a negotiation and hear a number that's lower than their peers, they can call bullshit and make a take it or leave it counter.

Companies get upset about unions because they're not paying fair pay right now and know it, and if everyone in a trade demands the same minimum rate then they have to accept it.