SIGACT is an international organization that fosters and promotes the discovery and dissemination of high quality research in theoretical computer science (TCS), the formal analysis of efficient computation and computational processes. TCS covers a wide variety of topics including algorithms, data structures, computational complexity, parallel and distributed computation, probabilistic computation, quantum computation, automata theory, information theory, cryptography, program semantics and verification, machine learning, computational biology, computational economics, computational geometry, and computational number theory and algebra. Work in this field is often distinguished by its emphasis on mathematical technique and rigor.Theoretical computer science in Europe has a much different balance, putting as much or even more emphasis on automata and logic as it does on algorithms and complexity. So from that point of view (a view shared by Moshe who has strong ties to CS logic) SIGACT does not cover the full range of TCS.
The term "theoretical computer science" just doesn't have a universal meaning. Neither definition is right or wrong, though we all have our biases.
Why does TCS have such a different meaning in Europe and the US? A different research culture and relatively little mixing. Very few North Americans go to grad school, take postdocs or faculty positions in Europe and very few Europeans go the other way, and those that did tended to do algorithms and complexity. Until we started to see web-based archives in the mid 90's, distribution of research between the Europe and US went quite slowly. Things have changed since but by then the different notions of TCS have been set.
SIGACT fully acknowledges that it doesn't do a good job covering the logic community and it has always strongly supported SIGLOG, the special interest group for logic and computation.
I would love to see joint events between SIGACT and SIGLOG. LICS should be part of FCRC with STOC and Complexity or hold a co-located meeting in other years. But SIGACT does do a great job representing the theoretical computer science community in North America.