There is a spectrum of goods and services suppliers, from luxury to essential. These gentlemen wanted a wedding cake. It's not essential and they may have had time to find an alternative supplier that, as you say, wasn't a jerk.
What happens when it's the pharmacy or grocery store? Or their kid's hockey coach or piano teacher? Or the only car mechanic in town?
I generally believe in a business' right to refuse service but I wrestle with situations like this.