The young have picked up somehow, perhaps via social media, that the their elders view Sharia with great disdain and some fear. What better way to send us a message that they are angry with the society we leave them than to express positive sentiments about something we see as anathema?
They are trying, with some success, to get our attention. I bet most of the non-Muslims expressing approval of Sharia have next-to-no idea of what it entails.
Be glad they aren't doing what French immigrant 'youth' are currently doing, namely rioting, looting and vandalism. Doing so is a French tradition.