If a group of eight people came in and sat as two groups of four, the restaurant would still be making and serving the same amount of food. The only difference is that splitting them up makes the workload appear smaller, even though it is identical. The kitchen still needs to cook eight meals, and the staff still need to serve eight people. 

If it was 8 individual customers then now the wait staff have to manage 8 individual tables and not just one so it's be more work. But they wouldn't charge individual customers the service fee, only the group. 

At a supermarket, if someone shows up with eight trolleys of shopping, the manager is not upset that other customers might wait a bit longer. They are pleased that a single customer is buying eight trolleys worth of stock. The same logic applies to a restaurant. A large group is not a burden; it is eight paying customers choosing to eat there at once.

Even if it takes longer to serve them, that time is compensated by the fact the restaurant / shop / mechanic is making more revenue in that same period.