My understanding of the ACT rationale is that even trying to change dysfunctional cognitions will dignify them in an undue way and reinforce "cognitive fusion" (i.e. taking thoughts/memories/images as literal truth). The whole thrust of ACT interventions is on undermining the client's "enmeshment" with his/her own thoughts and cognitive restructuring, therefore, is regarded as counterproductive.
Also, cognitive restructuring work, implicitly or explicitly, "colludes" with the client's control agenda (here: control of content of thought in order to control content/topography of emotional experience and associated behaviour) while ACT is focused on finding ways to "get on with life in a direction that is determined by the client's personal values" in the continued presence of aversive private experiences (i.e. thoughts, affect/mood, body sensations). In other words, in ACT the client's agenda of : "In order to lead the life I want to lead I will have to change my feelings/body sensations through changing my thoughts first" is sought to be replaced by the new agenda: "I will put my energy into actively trying to lead the life I want to lead while accepting that certain private experiences (cognitions,affect/mood/body sensations) may or may not be present.

In yet other words: Much like in DBT (opposite action principle) the idea is
1.that affect/mood/cognitive states will change if behaviour changes
2.the long-term satisfaction/dissatisfaction with life does depend more on one's actions than on cognitive content
3. trying to control private experiences (and cognitions in particular) is likely to produce paradoxical negative effects and leads to a rather maladaptive (ego-centric) fixation with these experiences at the expense of participating in one's actual life/context/environment
4. Therefore radical acceptance (achieved through building of mindfulness skills and cognitive defusion exercises rather than disputation and intellectual discussion) is more likely to produce a "fading" effect and flexibility in handling one's private experiences

In my personal practise I sometimes use cognitive restructuing techniques in order to "prime" mindfulness/acceptance work as it sometimes helps clients to just consider the possibility that their cognitions are not "made" of the same "stuff" as their actual/physical context.

(Sorry, that this reply took many months to be written)