I saw this on the way to work the other day. This is a typical union tactic in Chicago to attempt to cow a building owner or someone doing redevelopment to force them to use expensive, unproductive union labor.
On the other hand, even the most cursory walk through a neighborhood like Bucktown with lots of smaller redevelopment shows that there are many building sites exclusively employing illegal immigrants. You hear a lot of spanish spoken, plus polish and russian, or very authentic irish accents.
On a personal level a good friend of mine has an in-law who is an electrician and got fed up with all of the union BS. However, if you aren't in a union, finding business through subtle tactics such as the ones employed above, you are in competition with the completely unregulated illegal labor. As a result, he has trouble finding work that is appropriate to his level of skills since he is competing against people who are willing to work for very little (relative to the danger, etc... involved in construction).
The unions can't picket all of the smaller building redevelopment sites, but they do picket the bigger sites. Thus the small sites pretty much go non-union with illegals.
In a better-run city, instead of having two unappetizing choices of heavy handed, unproductive union labor and totally unregulated illegal labor, there would be something in the middle - maybe a productive, partially regulated work force, with reasonable inspections to ensure quality work.
Not in Chicago.