I've had bugs with some EM event drop containers in the past that eventually led to items becoming stuck in them.
That usually happens when the container has another effect, like Smoot said. At some point, it seems someone decided to make some of these EM drops not actually work as a container, even tho it looks like they should (And I think some do still let items get dropped into them mistakenly). It all depends on the graphic or the specific EM reward.
For example:
Example 1:
^ Used to be equipable AND work as a container. At some point a patch made it so ANY item with that graphic could no longer be used as a container. These used to work, but now dont. When double clicking this container, it says "You cannot use this" and nothing opens.
Example 2:
^ This item shows its a container + the container gump opens when you double click, but it seems it was made to not actually allow you to drop items into it when it was created.
^ Same exact graphic, but from a different EM/Shard. Like above it shows its a container and it shows the container gump. This DOES allow you to drop items into it.
Example 3:
^ This item shows its a container + the container gump opens when you double click, but it seems it was made to not actually allow you to drop items into it when it was created.
^ Shows its a container but when double clicking it, it says "You cannot use this". Despite that, you CAN drop items into this, but you CANNOT open it to retrieve the item.
Example 4:
^ Doesnt show that it is a container, but when double clicking the item it opens a container gump (It does not allow you to drop an item into it).
^ This one does let you drop items into it, but youll have problems getting them out.
^ Doesnt show as a container, doesnt open gump when clicking it (as it has the puff of smoke effect)
Basically it all really depends on what the EM decided when making it (Or possibly the Devs as in example 1, although I think that one was due to an exploit). Hell there are evens some EM spellbooks that the EM decided to make that you can't even equip (While others with the same graphic ARE equipable).
This doesnt stop with just books, Im just a big book collector so its what I have on hand. Many EM items that are also containers go through these issues.
*Usually if your EM item wont let you open it to take an item out, you can put the item on a pack horse. That usually will let you open it to take something out.
The 2 books in OP's post never actually worked as containers.