Answer: Because they can! :)
Step-by-step explanation:
It's because the brown, hairy coconut that you're referring to is just the aged, or matured version of this: They are one, and the same. The first one appears hairy because the husk has been removed, which has also a brown or yellow color by then.
Subsequently, the question is, can you drink coconut water from brown coconuts? The warning label on each coconut describes how to treat the water within each. The water on the brown coconut is to be discarded; the water on the white coconut is safe for consumption.