In cases when you are drifting towards the barriers, yes you should look cause if someones there he's going to crash. It's not difficult to glance right for a split second before moving right, as was said you do the same in a car, Sagan is 100% responsible there's no other way of seeing it. Switching lanes in a sprint is dangerous and requires making sure of your action, in this case Sagan should have slowed up and waited for Cav to go through it's not Cavendish's fault Sagan is badly placed, he shouldn't have to break to let him through or worse crash.
sorry, comparing a bunch sprint situation with "normal day traffic" makes absolutely no sense at all ..... the sprinters are riding at max speed possible, within the limits of man and bike (would be comparable with dragster racing perhaps) ... there is no way you can look around and consider decisions - it's all about instinct, which you either have or you don't (that's also one reason why bunch sprints including non-sprinters, like in first week TdF stages, are that dangerous) .... if you look around too much, you'll loose your balance and by that way cause accidents ...
there are three things very dangerous in cycling: descents, cars/motorbikes running over you and bunch sprints ....
the only way you could make bunch sprints a little safer would be to mark lanes in say the last 300m and everyone (or his teammate) crossing any lane would be disqualified .... however I am not sure if the roads are broad enough to have say at least 4 - 5 lanes