[jacquesm]

It's something analogous to Occam's razor combined with the principle of least surprise applied at a very large scale.

Given our observation of the 'laws' locally the assumption that they hold universally is the simplest explanation. It would require something a lot more complex than what we generally observe for such laws not to hold universally.

The places where the laws break down (big bang, inside black holes (do they) and other extremities) are as far as we're concerned not places we are likely to visit and are nice examples of how forceful you'd have to be to get out of the set of laws that we observe locally. It's probably safe to say that any place where the universally observed laws do not hold are places where energy levels are in play that we'd do best to avoid.