I'll always remember how my German brother-in-law found that when he arrived in America, retail store clerks were always eager to offer help, but actually didn't know anything about the products being sold and were untrained and basically useless.
That said, other countries with unionized retail seem to be able to provide excellent customer service. So, maybe the problem isn't unions, but Americans (or Americans in unions)(or Americans in unions in a country where unionization has historically been waning). Or maybe your experience mischaracterizes the actual circumstances.