@Developers Responsibility
If the builder left a door-sized hole in the wall covered by a tarpaulin and didn't tell the buyer about it, then a burglar wandered in and made off with their new plasma TV, I would expect that there would be a pretty strong case to be answer there.
I think it's pretty much a question of degree. I wouldn't hold a builder responsible if the front door was fitted with a cheap lock that could be easily forced - I wouldn't expect anything expensive unless it had been explicitly stated.
The same goes for software - glaringingly obvious security holes suggest negligence on the part of the developer, but there will always be bugs and unforeseen circumstances where software can break down. The reality is that real-world software is not perfect because developers have to work to deadlines and within budgetary constraints.
The danger is that the government will pass some ill-considered law that holds software developers responsible for fixing every flaw, no matter how minor, that is ever discovered in a piece of software. That would put people out of business, as it would increase the cost of software by an order of magnitude.