Re: Religious Tolerance (Shias and Sunnis)
Because they believed that polytheistic practices (shirk) was going on in those shrines, and if you go through the religious texts of Islam which is agreed upon by the mainstream of Ahl-as-Sunnah (with exception of few Sufis, Barelvis), building shrines over graves is forbidden. And the Prophet of Islam has cursed the Jews and Christians because they converted the graves of Prophets to places of worship.
So what Saudis did was not actually against a particular sect, it was a generalized act against both Sunni and Shia groups.
In Islam we don’t tolerate practices of Shirk on where Islamic laws are to be implemented.