Seemingly when they do something like the following...
I volunteered for the Green Festival at the Denver Convention Center the first weekend of the month, and on that Friday I was on the staff that was checking in festival exhibitors and vendors. My partner was there to learn about festival organization and to interview folks for the Interdependent Project and she was also checking in exhibitors on the same shift.
The exhibitors (well over 200) were of all shapes and sizes in all respects and most were in the excited, semi-agitated pre-show frame of mind. We enjoyed checking in all of the good people and only one thing marred the process. Exhibitors and vendors were only allowed to bring in one armful into the exhibition area by themselves. Everything else had to be put through the loading docs at a rate of 45$ for 3 trips on a 3x5 cart (apx. 30 minutes of work).
I wasn't at all bothered by the amount of the charge that union workers were charging, but I was entirely bothered that they wouldn't let exhibitors and vendors to carry in their own booths. Many, many of those running booths were SMALL business persons, two or three people trying to scratch out a living in an environmentally friendly way (all participants were required to have an absolute minimum carbon footprint). Some had a lot of material for sale and/or exhibition where the loading charges could exceed 1000$. Even 45$ (for very small exhibits) was not insignificant and might represent several days of expenses. One gal who was running her own booth without help was nearly in tears, union personnel had physically barred her from entering with a second load (in her arms).
The event organizers had published the financial ground rules at the end of their 36 page exhibitor brochure but obviously some people didn't make it that far and were surprised by the charges. A few bitterly complained that they couldn't afford to vend if they couldn't bring in their own loads and that they wouldn't have come if they'd known. While one can certainly fault their powers of observation (i.e., READ EVERYTHING about your responsibilities), we should also recognize that there were probably some exhibitors/vendors that did read the brochure that weren't able to financially justify showing some worthy products or causes (non-profits) at the event.
I'm all for unions fighting for better working conditions for the masses, but shit like this is really working against public perception of unions. Musical artists can relate to this as most who don't make very much money touring are often forced to reduce their meager incomes by union rules that essentially force musicians to pay to play, even if they're small enough to do it themselves with a roadie or two.
So what's the deal? Can't there be union exceptions for truly small businesses? I suppose there might have been some back room dealing that isn't presented in my viewpoint of this story, but in any event it didn't shed a pleasant light on union behavior.