Critiquing the Cultural Revolution has been acceptable since 1981 when Deng published/authorized the party's second "Resolution on History," which laid out officially that there were "excesses" during the Cultural Revolution, and that they were primarily Mao's fault.

This is in line with a lot of Chinese/Communist political thought in general - the Party is never wrong. If the Party endorsed what is later viewed as a "wrong action," then it must be the fault of an individual who erred or was corrupted in some way.