To be successful under FPTP, party leaders have to adopt many faces to appeal to a broad set of voters. To gain the votes at the edges of the political spectrum, parties have to suggest that while some minor party might be better for them, only they have a realistic chance of getting anything done that's in your direction, that voting for the minor party would be a wasted vote as it would let the opposition in.
It's also why parties don't kick out "rebel" MPs, and in fact, you find they're some of the longest standing MPs. They are, in reality, paper tigers. They are tiny in number and pose no threat to the leadership. But they serve a useful purpose to the leadership of keeping the base vote, while most policies are enacted towards either the centre, or to political interests. They leave those in the "base" with a vague idea that they'll do something for them.
Cameron's decision to announce that he won't have an EU referendum is just foolishness. His MPs could have replied that "well, we haven't ruled out a referendum, it's under review" to potential escapees to UKIP. They can't now. And as the rest of their policies are a fag paper width away from Labour, there's going to be a lot less nose-holding that there would have been.