> How do they get away with that
Ancestry has a limited number of people, and limited funds. The idiots can get away with it because Ancestry doesn't check user-submitted trees. I could, if I was feeling puckish, put a tree there that showed I was the love child of Elvis Presley and Marilyn Monroe. (I'm not, of course. I'm the love child of Abraham Lincoln, the president, and Jenny Lind, the "Swedish Nightingale".)
That's one of the biggest reasons we top 10 all tell beginners not to trust the family trees there; use them as clues. (Some of the "official" records are wrong too. About 1/8 of the parents' birthplaces on the US Census from 1880 - 1930 are wrong. People go from being male in 1920 to female in 1920, and I found one in which the mother and daughter had switched names. There is a 1-year discrepancy in the 1900 census birth year and the SSDI birth year on at least 10% of the individuals I look up.)
> is there anything I can do to fix it?
You can write to the person who has the tree, citing sources.
You can put a comment on the individual, citing sources.
If the details are in an "official" record, like the census, you can add a correction, citing sources.