That's just a rule they made up for themselves. So they don't need to make something up to convince themselves to break their own rule.
(and if the point for wanting main events to have a belt would be that they fear it would look less like a real sport if they put a non-title fight as the main before a real title fight, ...well creating an obviously fake belt wouldn't help that issue)
And I don't get why I would enjoy that fight more if it hade some made up title associated with it.
However, I can absolutely imagine that it would bring in more dough and the UFC have probably contemplated this idea as they know that spectacles and belt automatically create more PR (even if it would partially be people complaining how silly it is, that still generates interest)