Because there are two distinct 4/50 covers in the US. There is the warranty, for defective parts and manufacturing (same as the rest of the world, although the term may vary). Then, there is the maintenance program that covers wear-out of all components, including brake pads, clutch disks, wiper blades, etc. The maintenance plan for wear out has nothing to do with warranty, but the two coverages are often blurred together by owners.

You can experience early-hour defective rotors, but not several times over an extended period, that is more likely related to usage.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue