(I'm not a native speaker of English, but this seems wrong grammatically...)

It's certainly wrong ... but whether it was a grammatical error or a typographical error could well be a moot point :-)

I don't think this proposal is a good idea... (or should it be "I do think this proposal is not a good idea"...? ;)

Heh ... nice point. In terms of "usage" there's no (or little) difference. But in terms of "logic" your question confuses the crap out of me.

Cheers,
Rob