Fundamentally, a wiretap is an intentional breach of security. It may be a desirable or even a necessary breach, but it is a breach nevertheless. Furthermore, the easier it is for the "good guys" to "break in", the easier it may be for the bad guys. The Greek cellphone tapping scandal is just one case in point.
There’s another, more subtle, problem: if your wiretap is done incorrectly, perhaps by relying on incorrect information, you may miss traffic that you’re entitled to hear (and should hear, to protect society).
The Protect America Act carries both risks. Matt Blaze, Whit Diffie, Susan Landau, Peter Neumann, Jennifer Rexford, and I have written an analysis of the dangers. It will appear soon in IEEE Security and Privacy; you can download a preprint here.