This doesn't take into account the realities of an E3 demo.
For one, the clever tricks that ease you in to the controls generally occur at the start of the game. It's not just the UI of the game that allow a first time player to be able to understand, it's the design of a level to teach you. This is often not the part of the game that an E3 demo wants to show.
The second point is that a lot of the more subtle stuff that hints to new players how to play may not even be in the game yet. These E3 demos are with an incomplete product, but the developers have slaved to create a section of the game that seems polished. However, they know their target audience for the demo are experienced gamers, not members of the general public, so it changes their development priorities accordingly.
For one, the clever tricks that ease you in to the controls generally occur at the start of the game. It's not just the UI of the game that allow a first time player to be able to understand, it's the design of a level to teach you. This is often not the part of the game that an E3 demo wants to show.
The second point is that a lot of the more subtle stuff that hints to new players how to play may not even be in the game yet. These E3 demos are with an incomplete product, but the developers have slaved to create a section of the game that seems polished. However, they know their target audience for the demo are experienced gamers, not members of the general public, so it changes their development priorities accordingly.