There are multiple definitions of fairness. FCFS might be considered a fair admittance rule, but not an occupancy rule. Consider processes in a computer system. Do we let one process hog the CPU indefinitely while others are waiting? No, most usable systems will context-switch when the current process exceeds its time quantum. "First in, first out" is usually a better rule for occupancy. Applied to coffee-shop seats, that would mean that people who haven't bought anything for a long time should lose their place to people who purchased something more recently. It's only fair.
There are multiple definitions of fairness. FCFS might be considered a fair admittance rule, but not an occupancy rule. Consider processes in a computer system. Do we let one process hog the CPU indefinitely while others are waiting? No, most usable systems will context-switch when the current process exceeds its time quantum. "First in, first out" is usually a better rule for occupancy. Applied to coffee-shop seats, that would mean that people who haven't bought anything for a long time should lose their place to people who purchased something more recently. It's only fair.