You are right in your first sentence. The second one, though, does not explain the first.
That is, because anyone can play with any mode the way they want.
You can`t beat adventure mode (beating the dragon in the end) for the only fact that you can`t place an ender eye and open the portal; not because the mode was made for exclusive use of adventure maps. I say that because a lot of adventure maps uses placing and destroying blocks as tools for puzzles.
It's actually really easy to make a nether portal frame using a bucket. Here is how. First dig a 1 deep pit that is 4X2. Make a wall along one side of the pit that is 4X4. Place water source blocks on top of the wall so that the whole wall has water flowing down it and into the pit. Then using the bucket, place lava source blocks against the wall starting from the bottom in the pit and working up the wall. The lava placed in flowing water instantly turns into an obsidian block. I make the pit 2 wide so that water doesn't want to flow all over, but just into the pit.
Can't you sort of NOT pick up blocks and NOT break them, making that impossible in the first place