Unfortunately, this isn't going to be changing. We have other tablet users who have multiple products on their tablets. This would cause all similar pages to group with similar pages (IE: pages labeled as Front Matter) instead of each product having all parts grouped together.
However, you can rename the parts whatever way you like on downloading (with the exception of the Core Rulebook and Bestiary, as this will break their hyperlinks) either in their containing folder or iTunes (if you're syncing to an iPad).