Labour just don't do EU democracy

Spotted in the middle of a long piece on Calling England was this

"Labour and Lib Dems thwart attempt to introduce "referendum lock" on future EU treaties. The Conservatives unsuccessfully tried to make an amendment to the Government's Constitutional Reform Bill on 19 January, which would have meant that any future Treaty transferring power to the EU be subject to a referendum. The amendment was defeated by 303 votes to 183."

I would interested as to the reasons Labour and the Lib Dems would give for this anti-democratic stance.