Sorry for your experience. But this is the problem with US visa process—it is not at all objective. It is very subjective and depends on the person interviewing you. Chances are that if you were allowed to speak to a completely different person in the embassy you would've had had a different outcome. A point-based VISA system, like that of UKs, allows you, to some extent, determine the outcome of the application process even before you step into the embassy.

I've had a similar experience, not in terms of the outcome (I got my visa) but in terms of the questions when I applied for a USA visa a couple of years back. And I am an Indian, and embassy was in India. But the questions were pretty similar. The only difference was I was applying for a tourist visa.

I've had similar experiences with foreign family members attempting to visit me, a US Citizen. It's just a roll of the dice, no matter how prepared you are, no matter how much evidence you have. It's all extremely subjective.

Despite what the US immigration authorities think, not everyone wants to give up a well-paying job and universal health care in their home country and move to America to take an illegal cash job as a dishwasher.

If you require a visa to visit a country where you need a visa, I would suggest contacting the organiser for an invitation letter. Organisers of international conferences are used to getting this kind of inquiry, and know how to deal with it. I have the feeling if the author had been able to produce an invitation letter in the beginning, he would have been much more likely to be able to attend the conference.

I don't know much about this stuff but I doubt it would work. For invitation generally there's some degree of responsibility put on the person sending out the invitation. If you are a random internet person buying a ticket they are probably not allowed to "invite" you because at that point it's like ... "why did you invite this person? do you know him?" .... "err ... because he paid $X for a ticket".