If they come to a judgement that the promises he stood on cannot be enacted, what are they to do? This is the reality of the situation we are in; no attempt to simplify it to labelling people as "liars" will resolve that.
The policies like allowing a referendum and offering to implement Brexit were both something which Conservative and Labour have maintained were a bad thing. However when UKIP came knocking on the electoral door, they both ended up offering these things.
This wasnt due to a sudden belief that a referendum or Brexit were in the interests of the people it was just to get re-elected. The evidence is that the politicians who are refusing to implement Brexit believed it was going to do harm before they stood for election.
It is not something that they realised after the votes were cast.