Pure speculation, but I can't imagine he would be able to vote No given his Libertarian platform, but didn't want to buck the Republican line here?
Here's my unofficial vote tally
Democrats: 34-4 in favor (four were not present and did not vote, including my Congressman, Stephen Lynch - boo to you Mr. Lynch!)
Republicans: 18-7 against (three were not present and did not vote, Paul voted present).
It's interesting that roughly a quarter of Republicans voted for this, considering they have been lock-step against anything the Democrats have proposed this year. It seems that this is one issue where (with the notable exception of Paul) the politicians actually voted their conscience?