And because, well, there's a sign, and, illegal or not, it would just be socially awkward to go into the one labeled for females.
The bottom line is that, bathrooms should be gender independent, because-
-Men should not necessarily be uncomfortable by the presence of women or non-binaries in the bathroom,
-Women should not necessarily be uncomfortable by the presence of men or non-binaries in the bathroom, and;
-Non-binaries should not necessarily be uncomfortable by the presence of cis-men, cis-women, or other types of non-binaries in the bathroom.
At the end of the day, human waste is human waste, and it's all headed for the sewer no matter the sex/gender of the human bodies that eliminated such waste; it breaks down and decomposes, and eventually its atoms and molecules are re-instated, little by little, back into nature in its due time.