Yeah, that's pretty !@#$% up. It's the main issue I have these days with parliament. THAT kinda thing should go to referendum; you really need proportional representation for it, not just the retarded "first past the post" system.
Besides that, even if an MP is elected on the platform to aim to keep MP's allowances down, they're not obliged to vote that way, and there's no way their electorate can hold them to it. :/