I like to have the opportunity to explain our reasoning before I disclose that it is nylon. We all hate plastic that ends up in nature, but we are using it in a product that is meant to last.
The reason we have 20% nylon is that it allows the yarn to be spun thinner, while keeping the needed durability. This makes these beanies thinner visually, but also a bit cooler, and makes them more suitable for all-year usage.
Other beanies are more suited to the winter, though they all work fine for windy cool days on the ocean, or high altitude hikes on a cloudy days.