Perjuries are not all created equal. Rod Blagojevich and Lewis "Scooter" Libby were both convicted of lying to federal agents, but they did so in order to impede criminal investigations into other suspected wrongdoing. Another baseball star, Barry Bonds, was indicted for perjury because he supposedly lied to a grand jury probing illegal drug trafficking -- testimony that could have allowed criminals to go free.
Various government officials prosecuted for lying to Congress about the Iran-Contra scandal were trying to suppress the truth and block congressional oversight on a matter of grave public concern. In instances like these, prosecution of perjury serves as a deterrent to conduct that interferes with vital government functions.
Clemens' supposed deceit, by contrast, came in a bit of congressional theater. The hearings were not necessary to formulate legislation -- and, in fact, no legislation came out of the process. The point was to grab the spotlight and convey the impression of action to gullible constituents. Congress was holding hearings just for the fun of holding hearings.
Reginald Brown, an associate White House general counsel under President George W. Bush, told The New York Times that the committee members were pushing the boundaries of their legitimate authority: "They did this to figure out whether Clemens or his trainer were telling the truth, and that is arguably not a legislative function. It's not Congress' job to hold perjury trials."
All this might be easier to see if the case involved a more sympathetic character than Clemens, whose plight is largely the result of his own gargantuan hubris. But a meaningless act of perjury should not become a criminal case merely because it was allegedly committed by a prize jerk.
In this case, prosecutors seem to be letting their pride and indignation lure them into a fight they would have been better off declining. Sort of like Roger Clemens.