not a lawyer, but i don't think you need a global government to have legislation covering contracts between two companies from different countries. I suppose the contract decides which juridiction will apply in case of trouble.

But if i understand your point, you see smart contracts just as some kind of tool that could be used in place of regular ones, whenever all the corner cases can be determined in advance and coded against, right ?

In which case we could have state decide which type of smart contract are legal under their juridiction and which aren't. If that's what you mean, that could be interesting indeed. I don't see how that would be applicable in practice : everything can be hacked or tricked, or fall apart, and human always are needed at some point in any autonomous system i know of.