It comes from this (Cory Doctorow, BoingBoing) post.
Basically, the ACTA is forcing the DMCA down the throats of all other countries.
Some of you know that around 2002, a professor gave a lecture in Europe on cryptology and current technology and reverse-engineered a particular technology (protected in the US under the DMCA) in order to analyze its cryptological safety (or something like that). When he flew to the US, he was arrested for violating the DMCA, even though what he did was solely to promote learning for his cryptology students. Of course, he would not have been arrested in the country where he gave this lecture.
So what does the ACTA mean? It means that he could have been arrested anywhere had the ACTA been in place at that time. Even though the reverse-engineering was done purely for educational purposes, as reverse-engineering, it was against the law.
The incidence of these stupidly-made arrests will only go up in the future.
What's worse is that while the DMCA at least makes clear its intentions in its names, the ACTA conflates counterfeiting with copyright, patents, and reverse-engineering.
Counterfeiting is a serious problem; many people get sick and die every day from shoddy-quality counterfeit drugs being passed off as the real product.
Reverse-engineering? If the US, UK, and Poland had not reverse-engineered the German Enigma Machine, we may have lost WWII.