In my area (Central Illinois) most national corporate accounts and all government projects require that you use union labor or pay "Prevailing Wage." That is essentially modeled on union scale pay, including most of the benefits.
That means that it costs around $65 - $75.00 an hour to put an installer in the field (including pay, benefits, payroll tax, general liability and workers comp). The insurance bill is a few dollars less for in-house workers, so a shop worker costs about $55.00 - $60.00 an hour. In contrast, a non-union shop can pay an installer $30.00/hr, making the cost to field that installer (pay, payroll tax, workers comp, general liability) closer to $40.00 - $45.00 an hour. A $20.00/hr shop employee, with lower insurance premiums, will cost between $25 - $30 an hour.
The client pays about double for union shop labor. Businesses that do not have a requirement to hire union labor or pay prevailing wage will usually hire a non-union shop. National corporations, government related jobs, and union supporters must use the union shops. Either model can be profitable if run right. Many union shops specialize in installation and maintenance, and most creative shops and small retail shops are non-union. Of course, many shops learn how to finagle the rules, but in the end the liability is just not worth the risk. Twenty years ago you could get away with a lot; in today's Information age you will likely get caught cheating.
It can be tough finding a union sign job. The pay is much better, the benfits are usually very good, and you have the protection of the union. You generally need to know somebody, or work your way into the business by going through the union apprentice process, getting your name out, and waiting for an opening. Non-union jobs are easier to find, and usually require a broader skill set and more knowledge of the sign business. Many excellent fabricators and installers cannot get into the union for a variety of reasons, and can negotiate a reasonable deal (but you will seldom get a pay package that comes anywhere close to what you can get through the union).