Islam compells muslim women to dress modestly, that is written in Qur'an. Qur'an does not mention the detail of how modest it should be. Qur'an does not mention the criteria of how modesty should be. Everything is all about interretation. There are so many interpretations about how women should dress.
Another fact about Islamic teaching that Islam does not alow compulsion in religion.
When a muslim tell other muslim how to dress, he/she is merely telling the interpretation he/she knows or the interpretation that he/she thinks the best. It is no problem in telling other people what we know right ? It is no problem to ask other member of society to follow what we think the best from other choices right ?
This is similar with when parents tell their daughter to not wear only G-string and sexy bra to school.
I know that what Islam teaches regarding how women should dress is in contrary with what my hormones want. And I know that what my hormones want is not always logical.