Lane straddling doesn't prevent people merging via the zipper rule. Rather, it prevents people already on the road from moving over to the merging lane, shooting ahead of a bunch of people, then merging again (slowing down everyone else by adding to the merging traffic). It's one of those behaviors where whatever convenience that person gets is greatly outweighed by the cumulative cost to everyone else.
Actually lane straddling doesn't even work to prevent someone from using the zipper rule. It prevents people from passing other people on the right (nobody should be passing anyone on the right most of the time anyway). But it doesn't prevent someone from merging into the other lane when the time comes. As long as you're pacing yourself vs. the speed of the traffic in the lane you want to merge into, everything's fine. Using the zipper rule doesn't mean you race ahead of everyone before merging, if anything it means waiting a little bit longer and maybe even merging behind a car you would otherwise be tempted to merge in front of.
Actually lane straddling doesn't even work to prevent someone from using the zipper rule. It prevents people from passing other people on the right (nobody should be passing anyone on the right most of the time anyway). But it doesn't prevent someone from merging into the other lane when the time comes. As long as you're pacing yourself vs. the speed of the traffic in the lane you want to merge into, everything's fine. Using the zipper rule doesn't mean you race ahead of everyone before merging, if anything it means waiting a little bit longer and maybe even merging behind a car you would otherwise be tempted to merge in front of.