Let me explain this to you, although I think that's pretty weird considering it's the law of your country and not mine. The federal government has jurisdiction over certain things, and the states have jurisdiction over others. Hence that whole "states rights" argument which has rolled along for decades-centuries, essentially a tussle over where the boundary between state and federal jurisdiction lay. However, irrespective of that, both federal and state laws must abide by the Constitution.
States have the right to set their own electoral systems. Hence for instance why they all have different laws governing elections. However, state law is still bound by the Constitution, the 14th Amendment of which states:
No person shall be a Senator or Representative in Congress, or elector of President and Vice-President, or hold any office, civil or military, under the United States, or under any State, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof.
So no, they are not taking Trump off the ballot under federal law.
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The 14th A. does not describe in detail what insurrection is, or what level of proof is required. Essentially, there are three: impeachment amounts to "because Congress says so", criminal law requires beyond reasonable doubt, and civil law requires a preponderance of evidence (i.e. "51%").
The initial judicial hearing in Colorado examined the evidence from both sides and concluded that - at least to a civil law standard - Trump engaged in insurrection. So, actually, he has been found "guilty" (albeit not to a criminal standard) in Colorado. However the court then declined to bar him from the ballot because it was not clear that the 14th Amendment applied to the presidency. The Colorado Supreme Court then disagreed by saying that they saw no good reason the 14th Amendment did not apply to the presidency, and therefore Trump should be barred considering the lower court found him responsible for insurrection.
When I say that SCOTUS can make shit up, they absolutely can and do. You have recently been complaining that Roe v. Wade was magicked up out of nowhere, basically arguing they can do this yourself. Another ruling I might draw your attention to is Bush v. Gore (2000): as is noted by many, the legal basis for ending the recount was as feeble and arbitrary as it gets - in fact the ruling specifically denied it should set precedent for other cases, which could be interpreted as the majority admitting that their reasoning was unsound. You can also check out all sorts of other areas where SCOTUS has at times simply overturned previous rulings (where Roe v. Wade again comes up). The simple fact any SCOTUS ruling is overturned tells us that SCOTUS can justify pretty much anything if they feel like it. There's no hard rule, no perfect interpretation. Nine people sit down one day and decide whatever the hell they please, and if a majority have certain ideological leanings, they will enforce their ideology on the whole nation, dressed up with some pretty paragraphs of legal waffle to make it look reasoned. At least, until a majority with other ideology comes along a few decades later.
SCOTUS certainly have reasonable grounds to set a good precedent for the evidential standard required to decide whether someone committed insurrection, and a criminal standard (thus overruling Colorado's supreme court) is very reasonable. They could, if they like, insist on impeachment before a presidential candidate is barred and find grounds to argue it. They could also just say states can decide this for themselves by their own internal processes (which might include civil standard), and provide reason. They could agree with the initial ruling that the 14th A. does not apply to the POTUS, or alternative agree with the CO supreme court that it does, and both could be perfectly easily reasoned. They can do absolutely fucking anything and make it stick. That's what any countries law amounts to in the end: the opinions of the people on the highest court in the land. If they are biased, corrupt, incompetent it doesn't really matter: that's your nation's law.