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.