Because she once played a starship captain on one of the "Star Trek" spinoffs, sci-fi fans in the Twitterverse are apoplectic, asking how an actress with a credit like that could participate in a movie like this. Those fans are confusing Hollywood with reality, a common mistake amongst the instant-anger set. They've forgotten that while Mulgrew played a space traveler on television, she is not a scientist (not that you have to be a scientist to know how our solar system works).
Her TV role doesn't exclude her from having all sorts of bizarre beliefs, and starring in a space soap doesn't require any actual knowledge of how the universe works. As long as you hit your marks, say your lines, and don't throw the production into overtime, you can buy into any garbage beliefs you like, especially in a country where Tom Cruise and John Travolta are still big stars despite their devotion to Xenu.
On the other hand, I'd better not see the nitwit who produced this trash documentary show up in a congressional hearing opposite
Lawrence Krauss or
Neil deGrasse Tyson. I can't even calculate the ratio on that false equivalence.