Much in the same vein that you can vote on questions that are closed (and any existing answers to such questions), I see no reason to restrict closed questions from having accepted answers.
Closure is not always permanent. Sometimes it leads to deletion, sometimes it leads to revisions that result in reopening, and sometimes the closure was erroneous and it simply gets reopened. In the foremost case, the "bonus" for acceptance is lost. In the latter cases, if the answer is sufficient with or without a need for revision on the question's part, it changes nothing on the merits of the answer towards the author's needs. Sometimes, even, it takes such an accurate answer that strikes the untold heart of the intended question, that leads to revisions for reopening.
As such, I find it'd come off as little more than arbitrary to hold off acceptance on a closed question. Closed is meant to prevent answers accruing, as the question appears to be non-belonging. It's deletion that truly marks this state, and as mentioned deletion will revert such reputation changes.