They can't stand the idea that some people might actually take their own priorities into account, instead of submitting to a homogenizing Cause. Considering how Bookchin came to anarchism from more authoritarian forms of leftism, it seems as if some of those tendencies likely never died off within him.
Like the Calvinists & capitalists before him, he believed in the devotion of one's life to the "revolution", his vision of heaven that supposedly justified the continued ignorance of one's own desires to support an abstraction. They wanted heaven or the rational accumulation of Capital, he wanted a cybernetic utopia, but like his forebearers, the only way to get there was from the dismissal of daily life, of subjectivity. The " lifestylists" chose to oppose his ideals, and as such recieved his fiery polemics in Bookchin's Pravdas.