Biden's DOJ demanded that SCOTUS rule on this quickly, so they did. Now that they didn't rule in Biden's DOJ's favor, now you complain.
It was based completely on the constitution AND based on past rulings. (see Nixon Vs Fitzgerald and Mississippi vs Johnson from 1867). Article 2 Section 4 states that a president may be "subject to prosecution" only when found guilty in an impeachment and removed from office." Trump was found innocent, thus he is immune.
We went 230 of our 235 years without prosecuting a former president for what he did while in office because we knew that the president had immunity for official acts. You CANNOT have a president constantly worried that something he does while president will be prosecuted years after he is out of office and the next administration decides he did something they don't like. Basically, the left's behavior against Trump these last few years necessitated the SCOTUS to make it clear that the president CANNOT be prosecuted for things he did officially while president. Had you just left Trump the fuck alone, we wouldn't have to have SCOTUS step in and spank the ass of democrats like this.
You do realize this protects your cho-mo-n-chief Biden too, right?
It was a very fair, right down the middle decision, good job SCOTUS. Immunity for official acts but not personal ones, sounds good to me.