The ramifications of not signing would be extremely hard.
For example, one thing to do would be to downgrade the credit rating of said country, raising its spread and interest rate. Remember, the companies that do the rating are all US companies. Then governments would be unable to borrow money from foreign and domestic banks, leading to a catastrophic economic spiral in said countries.
This would be like a death sentence for politicians, because they will be associated with the catastrophy, and they would not be elected again.
So, the politicians are willing to sign things like ACTA, in order not to jeopardise their career.