Earlier quoted context omitted.
If they overshoot, there's always motion to suppress evidence and associated consequences (fruit of poisonous tree, grounds for appeal, etc). I agree it risks being problematic, but allowing a blanket unsearched conduit for physical evidential items from defendants is problematic, too.
I'd argue there should be a line somewhere. There's cases where the authorities may have reason to believe that something is not protected attourney-client communication and intercept it, but at the same time they should not be allowed to just intercept anything because it could be evidence disguised as communication.
That same "foregone conclusion" doctrine should apply here too. IMO (but IANAL) the court's argument here could be easily applied to interception/seizure of any communications between client and attorney, which is absurd.
edit: I guess by "the court's argument" I mean the mostly-implicit notion that "filter teams" are a sufficient mechanism to obviate any concerns over the government intercepting privileged communications. Most of what I read seemed to be an implicitly (again with the implicitude!) ex post facto justification of seizing the package based on the fact that it did contain non-privileged materials, and said materials would have been subject to discovery anyway.