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I don't think the issue is whether it's illegal or not. People will do illegal stuff on the internet, knowing that it's illegal.
The main problem is for those of us who watch it on American television don't want it spoiled, it ruins the show for them.
It's almost impossible to enforce the no-talking rule, it would be better for them to just ask people that have seen it not to spoil the episodes and wait just until everyone else has had a chance to see it til they discuss it.
That would be fair to people who obey the law.
But people, it's not hard to keep from talking about something on the internet. It's not like speaking aloud, where you can blurt it out. But posting, hell, you have to click several times to post something and in that time you can stop yourself.
If you want to talk about it, make a thread asking if they have seen it. For those who post that they do, PM them if you want to talk about it. Or better yet, if they can and you want a multiple person discussion, take it into a house in towns. There are so many options for not spoiling people, it's so easy.