A user can create a form for the users to fill in by adding input fields etc. They want to be able to preview this form and see how the fields look like for the user. They have no control over the submit button and cancel button. Should those buttons still be visible in the preview even though those buttons do nothing or should they be hidden? Disabled?
By all means, Preview means to show what user will see when they act on. It's like a final rehearsal before doing the actual action. If Preview is missing some elements, it misses the point. What if buttons don't look like user wanted? Or located not where they wanted? Or named not like they wanted? Yes, they have no means to change that, but giving them a hint of how the final form will look like is a good UX practice. And the buttons should not be grayed out or somehow else visually affected. We want the preview to be a carbon copy of actual form.
If you like to look at an example of a good preview, try Facebook ads. You will see your ad as other users will. There are all kinds of buttons, images etc, but none of them are clickable.