He has a constitutional protection from cruel and unusual punishment. If there's no reason to believe he's dangerous to society, and he's presumed innocent, and we can use a bond to ensure he'll appear, then there's no reason to hold him.
That is really the crux of the matter, and where people are (reasonably) disagreeing with you. Does a $4M bond (from his parents' assets, not his) and a $246M IOU really ensure SBF will show up in court rather than fleeing?
Reading about it a bit, it seems many of them are simple to remove (not sure what kind of monitor SBF is wearing), but that indeed, it will be detected immediately.
Having someone pay attention is also all that you need to prevent the world's most wanted man from hanging himself in a federal prison in Manhattan.
Cruel and unusual punishment does not apply here, and isn't really clearly defined regardless.
I personally agree that not holding him is good fwiw.
However, it also is horribly inconsistent. People who are guilty of driving while black, or non-violent drug offenses, or so on, are given much less generous treatment when it comes to unsecured bail, and when it comes to evaluating if they're a flight risk.
There's several factors at work here, but the most significant is that most offenders, especially most low-level offenders, are charged in state systems, and most (but not all, and this is changing, erratically) of the state systems rely much more heavily on financial conditions, and on secured bonds as the primary condition, in part due to very intense lobbying by the bail bonds industry to state governments (both directly and using astroturf groups to lobby the citizenry when their position is threatened.)