Restrictive elements are modifiers or appositives that restrict the meaning of the nouns they follow. Because they are essential to the meaning of the sentence, they are not set off with commas.
The modifier who use performance-enhancing drugs restricts the meaning of Skaters and is therefore essential to the meaning of the sentence. Putting commas around the who clause falsely suggests that all skaters use drugs.
Since Mead wrote more than one book, the appositive contains information essential to the meaning of the sentence.
Exercises:
Misuses of the comma 1
Misuses of the comma 2
Related topic:
Nonrestrictive vs. restrictive elements