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However, given the nature of how it was added, it's possible Obama didn't even know it was in there when he signed the appropriations bills it was slipped into -- as numerous Congressmen didn't know it was there when they voted for it. Instead, it seems the blame lies with another Democrat, Sen. Barbara Mikulski, chairman of the committee that added the provision anonymously (something I didn't even know was possible!).
In addition to being a horrible law (no company should ever be excused from liability over their products' health risks!!), it sets an extremely dangerous precedent in the Senate where provisions can be slipped into larger bills with little risk of being noticed (and thus easily passed without scrutiny), and zero accountability to the senators behind them (in this case we don't know who wrote the provision, only what committee it was in when it was added).
Interestingly, if Obama had followed through on the first promise he broke as President, "sunlight before signing" could very likely have revealed this provision to the nation before it was signed into law, and he would have had the chance to veto it over that. So while it's easy to excuse him for not knowing about it due to the nature in which it was added, the speed in which it was later discovered proves the value of "sunlight before signing", and puts even more shame on Obama for breaking that promise within hours of being sworn into office.