Your first sentence implies that the interviewee wasn't acting professionally. I didn't get this impression from the post. It sounded like he just politely tried to end the interview process. Without more details and perhaps both sides of the story it's a mistake to label his behavior as inept.
You make a good point about moving careers forward. Gaining a useful industry contact is probably worth putting up with just a few hours of unwanted technical interviews (which are good, free practice like you said). This is true even if the culture is not a good fit. However the company was hostile and rude. They should have calmly and casually abided by the interviewee's request then followed-up with a non-confrontational email. The company's behavior during the interview process (when they should be on their best behavior) makes me highly doubt their value as a useful industry contact going forward.
You make a good point about moving careers forward. Gaining a useful industry contact is probably worth putting up with just a few hours of unwanted technical interviews (which are good, free practice like you said). This is true even if the culture is not a good fit. However the company was hostile and rude. They should have calmly and casually abided by the interviewee's request then followed-up with a non-confrontational email. The company's behavior during the interview process (when they should be on their best behavior) makes me highly doubt their value as a useful industry contact going forward.