Why does the Torah attribute the defiling of Dinah to the people of Shechem, when it was only Shechem who was guilty?
Seforno: Had the people not considered rape a normal thing to do, Shechem would not have raped Dinah. This renders them accessories to the crime. 1
Ohr ha'Chayim: Anshei Shechem helped him and counseled him to do what he did.
Ha'amek Davar: The episode occurred through the gathering of the local girls, and Dinah came out to see them. 2
In fact, it seems to have been common for the leaders to take, by force, the girls of the ordinary people as wives or concubines, as we saw in Bereishis 6:2 (Seforno).
Pirkei d'R. Eliezer (38): Shechem brought girls around Yaakov's tent to play with lyres, to entice her to come out.
Why does it say, "Asher Time'u Achosam"?
Ohr ha'Chayim (to 34:25) #1: Even though they were liable for theft, and not for Arayos, Shimon and Levi would not have killed them had Shechem not defiled her.
Ohr ha'Chayim #2: They looted the city due to the rape; this was payment for shame.
Why did Bnei Yaakov take all their property?
Malbim: They took it as payment for shame due to the rape. Ohr ha'Chayim (to 34:25,27) - the Rambam (Hilchos Na'arah 2:5) says that we evaluate how much is proper for her father and her family. It is according to the importance of the sinner and the victim. All their wealth would not suffice for Bas Yaakov, the most esteemed in the world, at the hands of the lowly Shechem! Even though they judged them Chayav Misah, Kam Lei bid'Rabah Minei (one who is Chayav Misah is exempt from payment) does not apply to Nochrim (Tosfos Eruvin 62a, DH Ben).
