There has always been a fierce anti-labor streak in US politics. Even the first red scare in 1920's (before the cold-war!) associated all unions and pro labor moments and even speech as communist.
There is a feverish anti-union attitude even among people who have never even met anyone in one. If you ask, you will even find intense dislike of every foreign union without even knowing it's name or where its country is.
Take police unions for example. It is well documented that many unions are fervently against policies such as always-on body cams that theoretically should protect a (good) cop and the citizen. Fail that, they are against releasing body cam footage.[0] Police unions are also known to protect abusive officers. [1]
I'm not trying to claim that there isn't an anti-union attitude, but it is not simply associating them with communism.
[0]https://www.nytimes.com/2018/01/09/nyregion/new-york-police-... [1] https://www.theatlantic.com/politics/archive/2014/12/how-pol...
Some police unions do these things and need to be stopped. But that does not mean all unions in every field are bad. There are evil corporations too, are all corporations bad? Are the majority? Why does the public precive that differently?
Since the cold war, the communism claim has given way to universally negative portal: the sole presentation in fact is negative. Usually by tacitly implying an unusual behavior is typical and inevitable of every possible union.
The difference is Exxon runs ads of happy children running through flowery fields and unions don't. And pointing out a union's deficiencies is not going to loose sponsors, possibly the reverse.
That's quite untrue. In several European countries, for example Germany, U.S.-style "union shops" (which, unfortunately, are basically mandatory under the NLRA) are illegal.
And in many states, under deceptively named "right to work" laws, union-shops are illegal even if the employer agreed to them. Interestingly, while employers can not always require union membership as terms of employment, they can always require binding arbitration and non-competes which is the opposite of a "right to work"
I've known people who worked under the UAW and Teamsters unions. The amount people got paid in US auto plants for menial tasks was outrageous. $15/h+ 15 years ago for the most unskilled roles. Hiring was full of nepotism. You had to know someone to get one of these overpaid jobs. Outsiders were not welcome. The UAW continued to demand above market salaries despite companies declaring bankruptcy. Auto makers responded by moving to Mexico. Great job UAW!
As for Teamsters, their history is full of corruption. Dues payments were a job requirement. No dues, no job. They protected the worst workers from being fired, much like police and teachers unions.