Why can't a Muslim attend the funeral of his/ her non-Muslim parents?

Answer

To say the least, your thinking is the effect of gross deficiency of Imaan. A Muslim may not attend the funeral of even his parents who are non-Muslims for the simple reason that he is supposed to be a Mu'min, not a half-baked, hybrid known as 'moslem'. He cannot participate in the funeral of kuffaar because he is a Mu'min.

It is essential for Muslims to understand the 'Red Line' for safeguarding their Imaan. The Red Line may not be crossed in violation of the commands of Allah Ta'ala. Crossing the Red Line will be entry into the domain of Kufr and everlasting Jahannam.

Many Muslims of these times, due to convolution of their Imaan as a consequence of western education, embrace kuffaar in ways which jettison their own Imaan. Allah Ta'ala says in the Qur'aan Majeed:

"O People of Imaan! Do not take as friends your fathers and your brothers if they

prefer (love) kufr over

Imaan. Those from

amongst you who befriend them, verily, they are the Zaalimoon."

(At-Taubah, Aayat 23)

"It is not valid (i.e. not permissible) for the Nabi and the Mu'mineen to

pray for the forgiveness of the Mushrikeen even

if they are close relatives