Wouldn't there be a possibility to put these styles in an external stylesheet? As the editor itself makes use of external css for it's own styling, adding a new stylesheet for the elements it creates would not be very difficult, right?
I'm sure this is conscious practice that is done with a purpose in mind. I'm only unaware of its explanation.
Also: I noticed that Redactor uses italic-tags () to style text as italic. Isn't this bad practice in terms of semantics?