There was an ayah from Surah Maidah (verse 33) which they refer to,
The punishment of those who wage war against Allah and His Messenger, and strive with might and main for mischief through the land is: execution, or crucifixion, or the cutting off of hands and feet from opposite sides, or exile from the land: that is their disgrace in this world, and a heavy punishment is theirs in the Hereafter;
(they shall be killed or crucified or their hands and their feet be cut off on the opposite sides, or be exiled from the land.) `Ali bin Abi Talhah said that Ibn `Abbas said about this Ayah, `He who takes up arms in Muslim land and spreads fear in the fairways and is captured, the Muslim Leader has the choice to either have him killed, crucified or cut off his hands and feet.'' Similar was said by Sa`id bin Al-Musayyib, Mujahid, `Ata', Al-Hasan Al-Basri, Ibrahim An-Nakha`i and Ad-Dahhak, as Abu Ja`far Ibn Jarir recorded. This view is supported by the fact that the word Aw (or), indicates a choice. As Allah said.
The somali people didnt take shabaab as their ´´leader´´ rather they spread fear in the lands,so infact it is them who should be executed or their feet and hands cutt off on the opposite sides.