I just learned yesterday of this other case (see also here), also from this summer, in which police arrested Christians for evangelizing Muslims (outside of a mosque) and confiscated their camera. The police seemed to take it very much amiss that they were videotaping in the first place.
A few salient differences from the case of the Dearborn four:
--In Philadelphia, the police (originally, the UPenn campus police, eventually supplemented by Philadelphia's finest) expressly asked the missionaries to stop their street preaching and singing outside the mosque, and the Christians refused. The Acts 17 folks, on the other hand, were never told to stop and in fact were told, "You're fine" by the police shortly before they were arrested with no further directions or explanation.
--The Philadelphia police not only confiscated but erased the Christians' videotape evidence. This should be treated as a very serious matter.
Now, for those who think that I jump to conclusions, I will say this: I know less about the Philadelphia case than I do about the Dearborn case, and it appears that one of the people arrested in Philadelphia (Marcavage) has had a stormy relationship with the City of Philadelphia already in areas apparently unrelated to Muslim evangelism. (E.g., He won a lawsuit allowing him to preach near the Liberty Bell.) So it is possible that the over-zealousness of the police in this case is related more to dislike of Marcavage than to Islam.
It is, however, a relevant question: In point of fact, the Acts 17 people were not told to stop talking to Muslims and the Philadelphia group was. The Acts 17 guys bent over backwards to show that they were not, in fact, being uncooperative, and I would guess that if they had been told to leave or to break off a conversation, they would have done so. They were arrested summarily with no such opportunity, as the video evidence bears out. But is it really the case that the boys in blue can just come up to you and say, "You have to stop talking on a public street" and must be obeyed? If so, that is a bit disturbing. It's not that I don't believe that police need some flex and some discretion to decide--on grounds that may not always be subject to rigid codification--that people are being loud or disturbing or need to be "moved along." That kind of discretion and on-the-spot judgment makes sense to me. On the other hand, when it seems to be exercised arbitrarily or, to put it differently, to be exercised specifically against Christian witness or specifically in contexts where it merely serves Muslim sensibilities, I have to call foul. Particularly interesting in Philadelphia is the report (if you believe it) of the female officer saying, "I'm concerned about what's on the video" as the prisoners are led away.
Certainly it seems clear that the police over-reacted in this case. I have no doubt that the image of large numbers of police descending upon the place and grabbing the camera is a correct description. It even invites a certain amount of humor: "We need backup. We suspect they have a hymnal! And we heard something about the Sword of the Spirit!" More to the point, it seems plausible to me that sometimes police justify the characterization of people as "problem people" by means of an exaggerated response. By bringing in large numbers of police they imply that these people must require many police to subdue.
Update on Acts 17: It is still unclear whether the charges against them will be dismissed. The judge has extended the date for deciding. It appears plausible that if their case does go to trial it will depend on testimony that was obscured in the first flurry of other wild accusations against them (of inciting a riot, etc.)--namely the fantasy-based testimony of one Roger Williams (I note the irony in the name), a self-styled Christian who claims to have been "surrounded" by Acts 17 some time before a "crowd" gathered about them (which was the original reason the police gave for arresting them). His testimony is refuted by video evidence, though the video evidence is partly incomplete because he asked them to turn off their camera, and part of their interaction with him (at his request) took place with the camera turned off. In any event, it seems plausible that some part at least of the charges will be entirely dismissed. I assume that they have not yet filed a civil suit against the Dearborn police, et. al., because they are waiting to see first how the court will deal with the criminal charges.