First, according to this article one of the lawyers suing Google is alleging that a Google patent application for increasing the accuracy of location based services by intercepting data communications indicates that the Google Streetview monitoring was intentional. I find this unconvincing. Unlike many other countries, the United States doesn't have a requirement that a company exploit patented technology. Absent some other evidence of intentionality, the patent application proves nothing (and, of course, if there was other evidence of intentionality, the patent application wouldn't be necessary).
Second, and more interestingly, some observers (e.g., here) have stated that the lawsuits against Google may have no merit because the electronic communications privacy act has a safe harbor for intercepting communications which are publicly accessible. It's an interesting argument, but I don't know it's a show stopper. The relevant statutory provision is 18 USC 2511(2)(g)(i):
(g) It shall not be unlawful under this chapter or chapter 121 of this title for any person—
(i) to intercept or access an electronic communication made through an electronic communication system that is configured so that such electronic communication is readily accessible to the general public;
"readily accessible to the general public" is then defined in 18 USC 2510(16):
(16) “readily accessible to the general public” means, with respect to a radio communication, that such communication is not—
(A) scrambled or encrypted;
That definition is the reason I don't think the publicly accessible argument is a show stopper. As I noted here, at least one of the parties bringing suit against Google has alleged that Google engaged in decrypting the communications it intercepted. I don't know what evidence they have to back that allegation. However, at this point, it doesn't matter, since at this stage in the litigation a court is bound to accept the allegations in the complaint as true.
Whether they have enough to get through discovery is another question entirely, but one which won't be raised until Google files its answer and moves for summary judgment.