In vanilla, when you pick-block an item frame with any item in it, you get a copy of the item. PocketMine, on the other hand, gives an item frame, not an item in it. PocketMine also allows obtaining items that are not supposed to be obtainable in normal means (e.g. Portal). It also doesn't check whether players already have the item in their inventory.
Issue 1:
Issue 2:
Issue 3:
Small note: block-picking an _empty_ item frame should indeed give you an item frame.
Additional side note: The condition is not whether or not it's available to creative. For example, it's possible to block-pick command blocks.
I'm actually not sure what's the condition... After that, I thought it is the items that I can obtain from /give, but blocks like Nether Reactor cannot be obtained from it but still pick-blockable.
In other news, I found another inconsistency and added it.