isnt cargo aircraft "off limits" ? unarmed ?
Nope, nothing is off limits in an insurgency. In fact, the bigger the bang you can create, the more publicity you generate and the greater the levels of fear you can engender, thus hopefully forcing the opposition to the negotiating table.
Plus there is already precedence for this sort of thing. A Belarussian Il-76 was shot down going into Mogadishu in 2007 and an Israeli 757 was attacked with 2x SA-7 whilst departing Mombassa Moi International in 2002. In both cases, Al Shabaab were implicated.