The Tories want the referendum to fail - this is not a secret. They have tied vote reform to a second proposal, which they are (as discussed yesterday) explicitly framing around Westminster as 'We will shaft the Labour party by fixing boundaries'. Why are they being so blatant about this cunning plan? Obviously, because a negative reaction is what they want. They want to sabotage the AV referendum, so they are tying it to something they are explicitly framing as a partisan land-grab. (ETA - this tying together is in the framing of the legislation to allow the referendum to happen).
It's as if the Tories tied vote reform to a granny-punching proposal or something. If the bill is on a joint proposal to bring in AV and cut the number of seats in poor areas, then Labour MPs will vote against it. Meanwhile many Tory MPs will oppose the AV bit of the yoked pair. The Tories know this and they consider it worth it to sabotage the Bill which they want to fail.
I honestly expected the Lib Dems to play more clever, to make it easier for people like me to give them qualified support.
ETA - in my first wording of this post I thought I would be given a say on both proposals, and added 'Perhaps they don't want my vote in the referendum, in which case, fine.' I should more accurately have said 'Perhaps they don't want Labour MPs support, and feel they can win without it, and we shall see.'
*of course this all leaves out the Lib Dems who love what the Tories are doing, don't need any persuasion and will probably join a reformed Tory party soon