> Why would the US choose to cede authority to the ICC here?

For the same reason as any other treaty - the corresponding benefits.

> If the US wishes to discipline its service members, they still can and do.

That's not what the ICC is for. The ICC is for when a country won't do so when they should be.

> Under no circumstances should any country allow a foreign entity to decide what its military can and cannot do.

The US has a very long history of telling other foreign entities what they can and cannot do.