Purchased products can be returned to the non-purchased part of the list one by one. Just tap/swipe an item to have it back among your non-purchased products.
To return all the crossed-out items to the non-purchased part of the list, tap the menu button in the top right corner of the list and select "Restore purchased":
Tap "Restore" to confirm that the purchased products should be returned to the non-purchased part of the list, or "Cancel" if you've tapped "Restore purchased" accidentally:
"Restore purchased" option will be inactive if you have no crossed-out products in the list, i.e. if there is nothing to restore.