I think it's fairly typical.
So although the garage charges per hour, the costs of running a garage are largely fixed, staff have to be paid whether they are working or not, so the hourly charge will need to cover all of those costs, not just the 30-mins it might take for an oil change.
The previous example of the mobile mechanic with a much lower charge rate works because his overheads will be much lower and I assume is a self-employed one man business.