I don't see that it follows. They had to take care that privileged documents were not seized, since those would be intermingled in the office with any evidence of bribe taking or transfers of money. But I don't see where inquiry into legislative acts need be made. A bribe is a bribe even if the person taking the bribe never fulfills the terms of it. Thus the Congressman's voting record, and bill drafting record, which are public record anyway, need not be examined... excpet by the members of the public perhaps.
Per Gravel "those things generally said or done in the House or the Senate in the performance of official duties " are privileged by the Speech and Debate Clause. Those are the "legislative acts" it protects in the SCOTUS's eyes.
No one is arguing that the prosecutors aren't entitled to the evidence they want (except the perp of course) just over what procedures they must take in searching for them in the privileged papers they were hidden in by Jefferson.
It's not just a legal issue but also a political one. (Thanks again for the rely to my legal procedure question.)