Update: SourceTree 1.9.1

You can enable force push in Tools/Options/Git/Enable Force Push. After it is enabled you can check “Force Push” check box in the “Push” dialog.

Original answer

You have to do a force push, because topic branch is already published and you are rewriting history. Commits A and B from origin/topic are removed if you rebase.

You should make a merge if you don’t want to do a force push, specially if you have a team members already working on topic.

You can’t do a force push with SourceTree for Windows yet (see answers at atlassian forums).
You can vote for this feature here: https://jira.atlassian.com/browse/SRCTREEWIN-338