Islamic faith based schools want teachers to teach about Islamic tenets. A qualification of being a certain religious faith is not discrimination.
Teachers do not have to teach at Faith based schools. The public schools are available to individuals of all faith. Religion is not an issue at public schools. It is not taught, or expressed freely. This option allows for teachers of all religion, or even no religion, have a place to teach. A teacher should be qualified in the subject being taught. Each religious environment wants to promote that religion. Each individual religion has the right to have qualified religious teachers in their privately run schools.
The reason this issue has not come to light is most teachers do not want to teach at schools of a different faith. If religion does not matter to them, public schools are the better option. A comfortable work environment is more important to most teachers. If an atheist taught at a religious school, or Jew taught at Islamic, and so forth, the environment would be uncomfortable. A Jewish teacher might be uncomfortable looking at crosses and Jesus at a Catholic school. This is the reason Christians that are caught up in scandals quit their religious schools. Along with math and science, Faith based schools promote their religious