style div not showing in posts

Whenever I align an image for example, the editor wraps the image in a div with class aligncenter.

But when I view the post/page the div is stripped out.

How can I prevent this? And why are the align buttons there in the first place?

Thanks

Edit Ok, what? in my editor it doesn't input divs but dl and dt for styling. However it doesn't show on the actual post/page.

<dl id="attachment_45" class="wp-caption aligncenter" style="width: 906px;" data-mce-style="width: 906px;">
   <dt class="wp-caption-dt">
      My image is here
   </dt>
   <dd class="wp-caption-dd">Caption</dd>
</dl>