git log shows the AuthorDate rather than the CommitDate, so SourceTree should probably do the same.

The AuthorDate differs from the CommitDate when a commit is cherry-picked or a branch is rebased. For example, git log --format=fuller shows the following for a commit that I cherry-picked the following day:

Author:     Chris Bandy <cbandy@sittercity.com>
AuthorDate: Wed Nov 30 14:19:19 2011 -0600
Commit:     Chris Bandy <cbandy@sittercity.com>
CommitDate: Thu Dec 1 20:52:35 2011 -0600

Also, I think the CommitDate matches the ancestry of a branch, making it largely uninformative in the Log View and file log.

When the CommitDate differs from the AuthorDate, it may be good to display both in the commit details/summary.