Scipio,
Some people are also equally abhorred that capital punishment is still implemented, or that the definition for treason in England still includes non-political or military matters like polluting the royal bloodline, and until 1998 it was a crime punishable by death under the Treason Act.
In Islam, Apostasy is equal to treason and is not limited to political or military matters. In the Islamic order of sacredness, Allah, then the Prophet, and then the Qur’ãn occupy the highest positions. Tawhid, and Prophethood form the constitution of Islam. Just as upholding and protecting the constitution of a country is sign of patriotism, and undermining it is a form of treason - in the same way open rejection of the fundamental beliefs of Islam by a Muslim is an act of treason. Apostasy, i.e., the public declaration of rejecting the fundamentals of Islam, has also negative influence on the Muslim society; it’s a source of fitna, public upheaval & anarchy. And that is why Islam has prescribed harsh punishment for it.