28 percent isn't bad considering only around 12 percent of the US is in a union and some organized groups like cops are at 50 percent.
My feeling is the teachers union is pulling numbers down as their numbers are large and we see them turning way far left, meanwhile the schools are failing. For most, the teachers union is what they are most likely familiar with.
So the 28% figure (within what I assume is the context of this thread since no sources were cited by the OP) is only those who have "a great deal" or "quite a lot" of confidence in organized labor.
What's not mentioned by pointing only to that figure is that an additional 46% of those polled have "some" confidence in organized labor. Which brings us to a total of 74% of those polled who have some confidence in organized labor or better.
That's a very different picture than the one drawn by suggesting that only those who have a "great deal" or "quite a lot" of confidence in organized labor trust organized labor.