The Bilstein warranty is for defects in workmanship and materials, not lifetime wear:

Krupp Bilstein of America warrants its gas pressure shock absorbers against defects in workmanship and materials for as long as you, the original purchaser, own your car. If at any time after purchase or installation, the shock malfunctions and Bilstein determines it to be defective, we will either replace it or repair it. There are only a few specific exceptions to this warranty. They include:

- original equipment shock absorbers - which are warrantied by the original equipment manufacturer,
- improper installation,
- racing, driving competition or off-road use,
- use on modified or other than Bilstein-recommended vehicles.