See this German link: http://www.heise.de/newsticker/meldung/Bundesregierung-sieht...
So I can understand her somewhat. If ACTA would've been yet another EU regulation passed without much fuss and recognition, it would've been better to focus on actual laws, where Leutheuser-Schnarrenberger still has to fight basically the rest of the government over data retention (I really dread the next government's actions here).
It's probably better this way, though. Any show that the public actually cares matters a lot.
Actually the ISP umbrella organizations published statements that they think ACTA bypasses democratic processes, and puts them into the position of police and judge all in one, and much more...
One basic problem with something like ACTA is that mostly it's "encouraging" countries to think and debate about "appropriate" measures and laws. A lot of the language is about "might" "should endeavor to" etc. – and it probably has to, or you might just clash with national legal concepts. So just signing it might imply a response from "Will do sir! Let's get the jackboots out!" to "Yeah, yeah, sure, we'll all have a nice chat about it…".
Still, signing it is a bad show of intent, a truly worrying symptom of current politics. It opens doors better left closed…
However there is a petition by law professors against ACTA coordinated by Prof. Metzger of the University of Hanover and signed by practically all leading German professors in the field of IT law. (See: http://www.iri.uni-hannover.de/acta-1668.html)