Why is it not permissible for a Muslim to accept a bequest from his Christian grandmother?

It is permissible for a Muslim to accept a bequest from his disbelieving relative, whether the latter is a belligerent (harbi), a dhimmi, or a musta'min, because the prohibition applies to inheritance, not bequests. Similarly, a bequest from a Muslim to a disbeliever is permissible. A Muslim's bequest to a dhimmi is valid, as is a dhimmi's bequest to a Muslim, and a dhimmi's bequest to another dhimmi. If a disbeliever bequeaths one-third or less of his property to his Muslim relative, the bequest is valid. A bequest to a belligerent in a land of war is valid according to Ahmad, Malik, and the majority of Shafi'is.