TED is a huge thing. It's a rather large honor to get invited and when you accept, you agree to participate to the fullest. Millions of people try to get invited to these because they make every effort to put tons of contacts in your presence to network with. But that must be reciprocated by you. If you leave, a part of that interaction between sessions (not to mention the no longer common info from the missed seminar) is missing from whomever might have wanted to meet you.
In short, it's a private invite with rules. Follow or don't accept. Just don't go bashing them for setting it up that way.
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