It's not ridiculous at all. Your cars dampers take a lot abuse during conventional, everyday driving. When they're working as intended, you don't even notice it. That's the point.
I'm glad someone besides myself has posted this chart. It's not just an advertising gimmick by Bilstein. Low end brand shock absorbers like KYB, for example, will degrade heavily before they reach 50,000 miles. This condition is aggravated by road condition and driving style. The majority of people drive like **** and road conditions all over the country continue to degrade. It's a recipe for dampers to fail early, and they do.
We have a '93 Toyota Previa van in my family that serves the specific task of hauling wheels and tires around. It has 260,000 miles on it and it has had more sets of shocks than I can remember. Recently, we opted for a cheap set of shocks and 20,000 miles later, they're completely blown.
Most people are not willing to agree with the 50,000 mile statement because they are too embarrassed to admit that their dampers are blown. Newsflash: Your dampers are most likely blown. There is a huge, noticeable difference between new dampers and ones that have, for example 80,000 miles on them. Granted some higher quality dampers wear slower and more gradually, they still degrade.