A trailing space can be added to the title, causing silly bugmail that says the title has been changed and the whitespace-only change is very hard to see, etc.
This accurately reflects the user's action. We generally try to faithfully do what the user says, not what we think they mean. It is much more confusing if Phabricator ignores your edit without explanation than if it faithfully reflects an edit exactly as you made it, by accident.
We could prompt the user about this or change the UI to make these edits more obvious or something, but this occurs so rarely that this doesn't seem worthwhile.