Still taking, allegedly.
So let's be clear on a few things.
When people talk about "union busting," there is a clear different between today and over 100 years ago. I know that people like to conflate the two things, but they are different. A century ago, you had, quite literally, pitched battles with casualties. Now, you have concerted ... but LEGAL ... efforts by corporations to keep unions from organizing.
There are, unfortunately, a number of law firms that specialize in helping these corporations. They all hire PI firms. That said, the actions of the majority of the PI firms that are hired (including Pinkerton) are entirely within the law. It's no different than hiring a PI to "spy on" your spouse that you think is cheating on you- distasteful, but lawful (so long as other laws aren't broken).
If you actually care about the rights of workers- AND YOU SHOULD, then instead of invoking old timey stuff that doesn't matter anymore, you should be pushing for structural change in, inter alia, the NLRA and state laws. Start with basic things, like the remedies afforded for violations. For example, did you know that if a company retaliates against a worker for organizing ... the remedy is just reinstatement and back pay? That's right- that's why companies feel relatively free to act against workers. The process is slow and the penalties are light.
And they know that people, the good people that comment on this forum? They are going to be "Oh, the PINKERTON! I PLAYED RED DEAD!!!!! THEY MURDER PEOPLE!!!!" And then go back to happily ordering their plastic stuff from Amazon.
If you want to affect real change, there's a lot you can do- educating yourself about what the actual issues are is a start. The issue isn't just what's unlawful ... the issue is what companies are legally allowed to get away with.