Is it really as kafkaesque as you state? You have to enumerate your planned activities in order to get a visa, but receiving said visa is in no way an affirmation that the enumerated activities are legal to perform? That sounds completely dysfunctional to me.
A US visa is simply permission to present yourself at a port of entry for admission, at which time you may questioned further by border control, and in rare cases denied entry.
Most B visas are valid for multiple entries over 10 years. The fact that you wrote a letter and brought it to your original visa interview (which may have been years ago, and likely wasn't even looked at by the officer, who in a busy consulate has less than 2 minutes to complete your interview) does not mean the US government has affirmatively granted you permisison to do everything on that letter.
I think getting taken away in chains and imprisoned is significantly worse than getting denied entry? I understand you're not saying the raid was conducted properly, but you're consistently downplaying the part that did the damage.
Inconsistent policy resulting in unpredictable denial of entry is bad, but a typical working professional may tolerate that risk. Inconsistent policy resulting in arrest at some random point during your stay followed by indefinite detention (how long would that have lasted without the high-level diplomatic response?) is much worse.