The Kacaan government was also seen as a threat (not just ICU) by Ethiopia/Isreal and a threat to Soviets ambitions to expand into the red sea , but the US even though they dislked the Kacaan regime preferred it at time over Ethiopia because they were trying to limit Soviet and communist influence in the region, they would switch over to Ethiopia's side in a heartbeat if they re-aligned themselves back with them.
Because before this the United States and Somalias relations deteriorated following the coup in 1969. Somalia went far in beyond to counter and limit US infiltration in the Indian ocean.
All of them had a problem with Somalia's Islamic leanings and our territorial protectionism btw, for the Soviet we weren't socialist or marxist enough because we didn't fully commit to the ideology in the real sense as we saw it as a tool to develop our economy and was Muslim oriented in our belief system and foreign policy. That's why they switched to Ethiopia's side the minute the Derg took over to profess communism.
When the Kacaan government collapsed it was golden opportunity by the US to implant their influence in the region and place a puppet government allied with them espousing secular capitalist democratic beliefs and for them to create a military base. The same way that they had done in Korea after the cold war, and throughout Europe and Japan after world war 2. There is a qoute that Walaalwhoops posted of a US army general professing this, i cant find it now though.
Believe it or not , the ICU was not a movement , it was various local courts throughout the country that united into one governing body.
The earliest courts came about not in 2006 but right after the collapse of the state 1991. It was undermined by the United States at the time and the various proxies they supported.
One of the earliest local courts symbol: The Islamic Court of Bermuda.
It's quite the irony that you mention AU peacekeeping, the Islamic courts were the reall peacekeepers and external forces did everything to dismantle them.
For example did you know that they raised their own military forces and police units via local businessmen?
This actually shows you how much this was a foreign funded/armed proxies raging war and havoc on the local populous , whereas the local populations, the sheikhs and businessmen were actively trying to combat them and bring order, rule of law and freedom.
See this poster it's pretty obvious how much legitimacy they had amongst Somalis.