Souter and Stevens were in the four who voted against the BSA. Both of them are consistently liberal. Stevens was appointed by Ford in 1975 when there was a huge Democratic post-Watergate edge in the Senate. Ford wasn't a conservative anyway, but even if he was he probably couldn't have gotten someone much more conservative than Stevens.
Souter was appointed by the first Pres. Bush, also when the Senate was solidly in the control of the Democrats. Souter's views on everything were kind of mysterious at the time, which was a large part of the reason Bush appointed him. He had a lot of trouble getting confirmed, ironically because most of the Democrats voted against him. He refused to say in his testimony how he'd vote on the Dem's litmus test issue, abortion. Bush's other appointee, Clarence Thomas, probably couldn't have been confirmed if he hadn't been black. As it was he had to go through hell you'll recall.
With the exception of Thomas and sometimes Scalia Republican presidents have consistently given us those kind of justices--- Souter, Stevens etc. (including Sandra Day O'Connor most of the time). Thomas was actually a happy accident for constitutionalists because I don't think anyone including the first President Bush realized just how dedicated he was to the Founders' vision. I think if George H.W. Bush had realized how different Thomas really was from Souter he wouldn't have nominated him.