Okay but let me just put this out there, American planes were being hijacked in the years leading up to 9/11 and everyone was still (understandably) shocked when 9/11 happened. Even we had stopped funding the Mujahideen, it wouldn't have been to prevent 9/11. That would be a completely insane thought process. Neither us nor the soviets ever expected Afghanistan to pose a credible threat to our homelands. People have questioned if Clinton could prevented 9/11 because it happened so shortly after his presidency and he plausibly could have seen warning signs.
It wasn't "he could have seen warning signs". It was a case of "Clinton DID see warning signs". Clinton mentioned Bin Laden as a thread during a speech in Australia on 9/10/01. How much he could have done to prevent it is another question.
Here's the catch, though; the warning signs didn't point to the actual events of 9/11. The threat from Bin Laden and the Taliban in 2001 pre-September must be put in the context of their then-most-devastating attack on the United States at the time: the bombing of the USS Cole. This occurred not in the US, but in the Middle East, specifically while the USS Cole was at anchor in Aden harbor. The attack was carried through by two suicide bombers in a fiberglass boat. Likewise, there had been threats to the World Trade Center complex before 9/11 by Al Qaeda, but the chief context for this was the failed bombing in 1993 which used a truck and was carried out without connection to Bin Laden. Similarly, the greatest act of terrorism on US soil before then was another truck bombing, this at Oklahoma City by a domestic terrorist, and the greatest single attack overall being another truck bombing in Beirut 1983, aimed by suicide bombers of unconfirmed (typically presumed to be Hezbollah's precursors) origin at barracks for US soldiers operating with the multinational force. Carrying further, hijackings of large passenger craft, while not unheard of before 9/11, were not typically used for suicide attacks. Most often, a hijacked airliner would be landed at some third-world location with the passengers and ransom demands sent out; they were used for fundraising and/or prisoner exchanges, rather than as weapons. Attacks on airliners tended to be directly aimed at the plane itself, as in the Lockerbie case. This unfortunately played a significant role in how the 9/11 hijackings were handled, as I recall: there was an expectation before the first attack went through that the hijackers would open negotiations and that the passengers could be safely released. While a threat could be ascertained from Bin Laden and Al Qaeda, ultimately, it appeared before 9/11 to primarily aimed at US assets abroad, particularly in the Middle East, and utilize primarily assets that could be obtained and controlled from the planning phase rather than assets seized as a part of the attack itself.
As for the question on whether American funding was decisive, it's actually worth noting in further support of arguments already posed that Saudi Arabia *did* in fact fund the Afghani mujahideen both during and after the Soviet intervention, and was in fact their second-largest bankroller after the US itself. In addition, Pakistan chipped in a bit as well as serving as a channel, Iran played its own cards during and after in opposition to both Soviet and Saudi influence, and China also provided training and other relatively minor support due to the ongoing Sino-Soviet split. It wasn't just "because reasons" as was so eloquently put, either; Saudi Arabia saw Soviet encroachment on the Middle East as a significant threat, cordially detested Soviet-supported Ba'athist Iraq in spite of their common Iranian enemy, and like the US, saw Afghanistan as a way to poke the Soviets in the eye while supporting Sunni factions against Iranian-supported Shi'a factions and atheistic Communists. In other words, I suppose you could blame the US for funding them, but as asserted, it isn't enough to say that US funding alone was sufficient to cause 9/11.