E X A C T L Y!
It is a movie. No more. No less. It’s just there for entertainment. Not sure why some don’t get that.
I don't know why you don't get why Christians are offended by the movie. It's a movie based on false advertising -- that the story of Noah would be the one told in the Bible -- designed to suck money out of Christians.
There is absolutely nothing wrong with telling Christians they shouldn't waste their money seeing it. If you don't like the criticism of the movie, don't read it.