The dissent points out,

It appears to be an open question whether viewing child p**nography on the internet is, in itself, a crime. See Martin, 426 F.3d at 77; United States v. Falso, 544 F.3d 110, 121 n.13 (2d Cir. 2008)(Sotomayor, J.)....

The dissent raises a question ignored by the majority? What if the purchaser got buyer's remorse the next morning?

Gourde [a 9th Cir. case]is different . . . from a person who actually mustered the money and nerve to become a member [of a child-p**nography website] but, the next morning, suffered buyer’s remorse or a belated fear of prosecution and cancelled his subscription. Instead, Gourde became a member and never looked back—his membership ended because the FBI shut down the site.

The majority found that the one month subscription alone constituted probable cause that the defendant not only viewed the images on the site but had downloaded them. That's a huge leap considering, as the dissent points out:

The undercover agent, however, “did not purchase a membership,” id., which would have enabled him to access the content of the website and determine whether it contained capabilities in addition to mere browsing. Furthermore, there is no additional information in the affidavit indicating that this was an egroup-type site....Nor were there other indications that it was an e-group or file-sharing website.

The dissent asks whether the majority would have reached the same conclusion had the offense under investigation not been child p**n:

I cannot think of any other circumstance where we have endorsed an invasion of a person’s privacy with so few facts from which to draw an inference that the intrusion would likely uncover evidence of a crime. What is the justification for such an unprecedented encroachment upon our constitutional protections? ....The answer is as obvious as it is unsettling. The majority’s conclusion is erringly shaped by the fact that child p**nography cases are particularly appalling.

Saving the best for last:

As reprehensible as our society finds those who peddle, purchase, and view child p**rnography, we, as judges, must not let our personal feelings of scorn and disgust overwhelm our duty to ensure the protection of individual constitutional rights. We must remember, as the district court observed, that we “must not deny the protections of the Constitution to the least of us. There is no such thing as a fair weather Constitution—one which offers the harbor of its protections against unreasonable search and seizure only in palatable contexts and only to worthy defendants.”

Update: Law Prof Doug Berman at Sentencing Law and Policy weighs in, as does Orin Kerr at Volokh.