Why do they top up brake fluid? The only reason the level should drop is because of wear on the brakes. If they keep topping it up you’ve then got to remove fluid when you replace the pads or it’ll overflow. As long as the fluid is above the minimum fill level they should leave it alone.
I suspect it is one of these "add-on" things where companies actually make their money - low cost but high profit option where they don't really do much probably just a token "top up" and brief check it is as should be.