The film analogy is particularly apt I think. If I'd been drip fed trailers of 'Gods of Egypt' which showed scenes with Sam Rockwell teaming up with Paddy Considine against robotic ninja cheerleaders, and featured an incredible scene depicting the destruction of Milton Keynes I'd be pretty hyped. If the time I found out this wasn't the case was after I've paid to see the movie and was walking out of the cinema then I'd rightfully be pissed and feel ripped off.
E3 demos and trailers are simply marketing tools, with the sole purpose of trying to get you to spend your money on the promise of what you've been shown and told. Of course there's going to be subtle differences (as there is with trailers to finished movies) - but the video game industry has an appalling track record. The number of games I've seen 'demo'd at E3 that ended up looking nothing like what was shown has just turned me off believing anything I see. It's pretty criminal all told, and a cynical way to get people to part with their cash.
I'm kind of glad there appears to be a bit of scrutiny now (Eurogamer never call developers out for this, and there's been little legal action around the practice), but its focus on No Man's Sky seems a bit misguided when there's much worse examples out there.